Cinchoo ETL - JSON Writer

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30 Sep 2022CPOL27 min read

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Simple JSON writer for .NET

This article discusses how to use ChoJSONWriter component offered by ChoETL framework. It is a simple utility class used to save JSON data to a file or an external data source.

Introduction
Requirement
"Hello World!" Sample
1. Quick write - Data First Approach
2. Code First Approach
3. Configuration First Approach
Writing All Records
Write Records Manually
Customize JSON Record
Customize JSON Fields
1. DefaultValue
2. ChoFallbackValue
3. Type Converters
4. Validations
5. ChoIgnoreMember
6. StringLength
7. Display
8. DisplayName
Callback Mechanism
1. Using JSONWriter events
2. Implementing IChoNotifyRecordWrite interface
3. BeginWrite
4. EndWrite
5. BeforeRecordWrite
6. AfterRecordWrite
7. RecordWriteError
8. BeforeRecordFieldWrite
9. AfterRecordFieldWrite
10. RecordWriteFieldError
Customization
Using Dynamic Object
Exceptions
Using MetadataType Annotation
Configuration Choices
1. Manual Configuration
2. Auto Map Configuration
3. Attaching MetadataType class
ToTextAll Helper Method
1. ToText Helper Method
Writing DataReader Helper Method
Writing DataTable Helper Method
Advanced Topics
1. Override Converters Format Specs
2. Currency Support
3. Enum Support
4. Boolean Support
5. DateTime Support
Fluent API
1. NullValueHandling
2. Formatting
3. WithFields
4. WithField
5. IgnoreFieldValueMode
6. ColumnCountStrict
7. Configure
8. Setup
FAQ
1. How to serialize an object?
2. How to serialize collection of objects?
3. How to serialize dynamic object?
4. How to serialize anonymous object?
5. How to serialize collection?
6. How to serialize dictionary?
7. How to serialize DataTable?
8. How to serialize JSON to a file?
9. How to serialize unindented JSON?
10. How to serialize conditional property?
11. How to serialize datetime in custom DateFormat?
12. How to exclude property from Json Serialization?
13. How to convert XML to JSON?
14. How to convert CSV to JSON?
History

1. Introduction

ChoETL is an open source ETL (extract, transform and load) framework for .NET. It is a code based library for extracting data from multiple sources, transforming, and loading into your very own data warehouse in .NET environment. You can have data in your data warehouse in no time.

This article talks about using ChoJSONWriter component offered by ChoETL framework. It is a simple utility class to save JSON data to a file / external data source.

Corresponding ChoJSONReader, a JSON reader article can be found here.

Features

Follows JSON standard file rules
Supports culture specific date, currency and number formats while generating files
Supports different character encoding
Provides fine control of date, currency, enum, boolean, number formats when writing files
Detailed and robust error handling, allowing you to quickly find and fix problems
Shortens your development time

2. Requirement

This framework library is written in C# using .NET 4.5 Framework / .NET core 2.x.

3. "Hello World!" Sample

Open VS.NET 2013 or higher
Create a sample VS.NET (.NET Framework 4.5) Console Application project
Install ChoETL via Package Manager Console using Nuget Command based on working .NET version:
- Install-Package ChoETL.JSON
- Install-Package ChoETL.JSON.NETStandard
Use the ChoETL namespace

Let's begin by looking into a simple example of generating the below JSON file having two columns.

Listing 3.1 Sample JSON Data File (emp.json)

JSON

[
  {
    "Id": 1,
    "Name": "Mark"
  },
  {
    "Id": 2,
    "Name": "Jason"
  }
]

There are a number of ways you can get the JSON file to be created with minimal setup.

3.1. Quick Write - Data First Approach

This is the zero-config and quickest approach to create JSON file in no time. No typed POCO object is needed. The sample code below shows how to generate a sample JSON file using dynamic objects.

Listing 3.1.1 Write List of Objects to JSON File

List<ExpandoObject> objs = new List<ExpandoObject>();
dynamic rec1 = new ExpandoObject();
rec1.Id = 1;
rec1.Name = "Mark";
objs.Add(rec1);
 
dynamic rec2 = new ExpandoObject();
rec2.Id = 2;
rec2.Name = "Jason";
objs.Add(rec2);
 
using (var parser = new ChoJSONWriter("emp.json"))
{
    parser.Write(objs);
}

In the above sample, we give the list of dynamic objects to JSONWriter at one pass to write them to JSON file.

Listing 3.1.2 Write Each Object to JSON File

using (var parser = new ChoJSONWriter("emp.json"))
{
    dynamic rec1 = new ExpandoObject();
    rec1.Id = 1;
    rec1.Name = "Mark";
    parser.Write(item);

    dynamic rec1 = new ExpandoObject();
    rec1.Id = 2;
    rec1.Name = "Jason";
    parser.Write(item);
}

In the above sample, we take control of constructing, passing each individual dynamic record to the JSONWriter to generate the JSON file using Write overload.

3.2. Code First Approach

This is another zeo-config way to generate JSON file using typed POCO class. First, define a simple POCO class to match the underlying JSON file layout.

Listing 3.2.1 Simple POCO Entity Class

public partial class EmployeeRecSimple
{
    public int Id { get; set; }
    public string Name { get; set; } 
}

In the above, the POCO class defines two properties matching the sample JSON file template.

Listing 3.2.2 Saving to JSON File

List<EmployeeRecSimple> objs = new List<EmployeeRecSimple>();

EmployeeRecSimple rec1 = new EmployeeRecSimple();
rec1.Id = 1;
rec1.Name = "Mark";
objs.Add(rec1);
 
EmployeeRecSimple rec2 = new EmployeeRecSimple();
rec2.Id = 2;
rec2.Name = "Jason";
objs.Add(rec2);
 
using (var parser = new ChoJSONWriter<EmployeeRecSimple>("emp.json"))
{
    parser.Write(objs);
}

The above sample shows how to create a JSON file from typed POCO class objects.

3.3. Configuration First Approach

In this model, we define the JSON configuration with all the necessary parameters along with JSON columns required to generate the sample JSON file.

Listing 3.3.1 Define JSON Configuration

ChoJSONRecordConfiguration config = new ChoJSONRecordConfiguration();
config.JSONRecordFieldConfigurations.Add(new ChoJSONRecordFieldConfiguration("Id"));
config.JSONRecordFieldConfigurations.Add(new ChoJSONRecordFieldConfiguration("Name"));

In the above, the class defines two JSON properties matching the sample JSON file template.

Listing 3.3.2 Generate JSON File without POCO Object

List<ExpandoObject> objs = new List<ExpandoObject>();

dynamic rec1 = new ExpandoObject();
rec1.Id = 1;
rec1.Name = "Mark";
objs.Add(rec1);
 
dynamic rec2 = new ExpandoObject();
rec2.Id = 2;
rec2.Name = "Jason";
objs.Add(rec2);
 
using (var parser = new ChoJSONWriter("emp.json", config))
{
    parser.Write(objs);
}

The above sample code shows how to generate JSON file from a list of dynamic objects using predefined JSON configuration setup. In the JSONWriter constructor, we specified the JSON configuration object to obey the JSON layout schema while creating the file. If there is any mismatch in the name or count of JSON columns, it will be reported as an error and stops the writing process.

Listing 3.3.3 Saving JSON File with POCO Object

List<EmployeeRecSimple> objs = new List<EmployeeRecSimple>();

EmployeeRecSimple rec1 = new EmployeeRecSimple();
rec1.Id = 1;
rec1.Name = "Mark";
objs.Add(rec1);
 
EmployeeRecSimple rec2 = new EmployeeRecSimple();
rec2.Id = 2;
rec2.Name = "Jason";
objs.Add(rec2);
 
using (var parser = new ChoJSONWriter<EmployeeRecSimple>("emp.json", config))
{
    parser.Write(objs);
}

The above sample code shows how to generate JSON file from a list of POCO objects with JSON configuration object. In the JSONWriter constructor, we specified the JSON configuration object.

3.4. Code First with Declarative Configuration

This is the combined approach to define POCO entity class along with attaching JSON configuration parameters declaratively. id is required column and name is optional value column with default value "XXXX". If name is not present, it will take the default value.

Listing 3.4.1 Define POCO Object

public class EmployeeRec
{
    [ChoJSONRecordField]
    [Required]
    public int? Id
    {
        get;
        set;
    }

    [ChoJSONRecordField]
    [DefaultValue("XXXX")]
    public string Name
    {
        get;
        set;
    }

    public override string ToString()
    {
        return "{0}. {1}.".FormatString(Id, Name);
    }
}

The code above illustrates about defining POCO object with necessary attributes required to generate JSON file. The first thing defines a property for each record field with ChoJSONRecordFieldAttribute to qualify for JSON record mapping. Id is a required property. We decorated it with RequiredAttribute. Name is given default value using DefaultValueAttribute. It means that if the Name value is not set in the object, JSONWriter spits the default value 'XXXX' to the file.

It is very simple and ready to save JSON data in no time.

Listing 3.4.2 Saving JSON File with POCO Object

List<EmployeeRec> objs = new List<EmployeeRec>();

EmployeeRec rec1 = new EmployeeRec();
rec1.Id = 1;
rec1.Name = "Mark";
objs.Add(rec1);
 
EmployeeRec rec2 = new EmployeeRec();
rec2.Id = 2;
rec2.Name = "Jason";
objs.Add(rec2);
 
using (var parser = new ChoJSONWriter<EmployeeRec>("emp.json"))
{
    parser.Write(objs);
}

We start by creating a new instance of ChoJSONWriter object. That's all. All the heavy lifting of generating JSON data from the objects is done by the writer under the hood.

By default, JSONWriter discovers and uses default configuration parameters while saving JSON file. These can be overridable according to your needs. The following sections will give you in-depth details about each configuration attribute.

4. Writing All Records

It is as easy as setting up POCO object match up with JSON file structure, construct the list of objects and pass it to JSONWriter's Write method. This will write the entire list of objects into JSON file in one single call.

Listing 4.1 Write to JSON File

List<EmployeeRec> objs = new List<EmployeeRec>();
//Construct and attach objects to this list
...

using (var parser = new ChoJSONWriter<EmployeeRec>("emp.json"))
{
    parser.Write(objs);
}

or:

Listing 4.2 Writer to JSON File Stream

List<EmployeeRec> objs = new List<EmployeeRec>();
//Construct and attach objects to this list
...

using (var tx = File.OpenWrite("emp.json"))
{
    using (var parser = new ChoJSONWriter<EmployeeRec>(tx))
    {
        parser.Write(objs);
    }
}

This model keeps your code elegant, clean, easy to read and maintain.

5. Write Records Manually

This is an alternative way to write each and every individual record to JSON file in case when the POCO objects are constructed in a disconnected way.

Listing 5.1 Writing to JSON File

var writer = new ChoJSONWriter<EmployeeRec>("emp.json");

EmployeeRec rec1 = new EmployeeRec();
rec1.Id = 1;
rec1.Name = "Mark";
 
writer.Write(rec1);

EmployeeRec rec2 = new EmployeeRec();
rec1.Id = 2;
rec1.Name = "Jason"; 

writer.Write(rec2);

6. Customize JSON Record

Using ChoJSONRecordObjectAttribute, you can customize the POCO entity object declaratively.

Listing 6.1 Customizing POCO Object for Each Record

[ChoJSONRecordObject]
public class EmployeeRec
{
    [ChoJSONRecordField]
    public int Id { get; set; }
    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    public string Name { get; set; }
}

Here are the available attributes to carry out customization of JSON load operation on a file:

Culture - The culture info used to read and write.
ColumnCountStrict - This flag indicates if an exception should be thrown if JSON field configuration mismatches with the data object members.
NullValue - Special null value text expects to be treated as null value from JSON file at the record level.
ErrorMode - This flag indicates if an exception should be thrown if writing and an expected field is failed to write. This can be overridden per property. Possible values are:
- IgnoreAndContinue - Ignore the error, record will be skipped and continue with next.
- ReportAndContinue - Report the error to POCO entity if it is of IChoNotifyRecordWrite type.
- ThrowAndStop - Throw the error and stop the execution.
IgnoreFieldValueMode - N/A
ObjectValidationMode - A flag to let the reader know about the type of validation to be performed with record object. Possible values are:
- Off - No object validation performed. (Default)
- MemberLevel - Validation performed before each JSON property gets written to the file
- ObjectLevel - Validation performed before all the POCO properties are written to the file

7. Customize JSON Fields

For each JSON column, you can specify the mapping in POCO entity property using ChoJSONRecordFieldAttribute.

Listing 7.1 Customizing POCO Object for JSON Columns

public class EmployeeRec
{
    [ChoJSONRecordField]
    public int Id { get; set; }
    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    public string Name { get; set; }
}

Here are the available members to add some customization to it for each property:

FieldName - JSON field name. If not specified, POCO object property name will be used as field name.
Size - Size of JSON column value
NullValue - Special null value text expect to be treated as null value from JSON file at the field level.
ErrorMode - This flag indicates if an exception should be thrown if writing and an expected field failed to convert and write. Possible values are:
- IgnoreAndContinue - Ignore the error and continue to load other properties of the record.
- ReportAndContinue - Report the error to POCO entity if it is of IChoRecord type.
- ThrowAndStop - Throw the error and stop the execution.

7.1. DefaultValue

Any POCO entity property can be specified with default value using System.ComponentModel.DefaultValueAttribute. It is the value used to write when the JSON value null (controlled via IgnoreFieldValueMode).

7.2. ChoFallbackValue

Any POCO entity property can be specified with fallback value using ChoETL.ChoFallbackValueAttribute. It is the value used when the property is failed to writer to JSON. Fallback value only set when ErrorMode is either IgnoreAndContinue or ReportAndContinue.

7.3. Type Converters

Most of the primitive types are automatically converted to string/text and save them to JSON file. If the value of the JSON field aren't automatically converted into the text value, you can specify a custom / built-in .NET converters to convert the value to text. These can be either IValueConverter, IChoValueConverter or TypeConverter converters.

There are couple of ways in which you can specify the converters for each field:

Declarative Approach
Configuration Approach

7.3.1. Declarative Approach

This model is applicable to POCO entity object only. If you have POCO class, you can specify the converters to each property to carry out necessary conversion on them. The samples below show the way to do it.

Listing 7.3.1.1 Specifying Type Converters

public class EmployeeRec
{
    [ChoJSONRecordField]
    [ChoTypeConverter(typeof(IntConverter))]
    public int Id { get; set; }
    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    public string Name { get; set; }
}

Listing 7.3.1.2 IntConverter Implementation

public class IntConverter : IValueConverter
{
    public object Convert
    (object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value;
    }
 
    public object ConvertBack
    (object value, Type targetType, object parameter, CultureInfo culture)
    {
        int intValue = (int)value;
        return intValue.ToString("D4");
    }
}

In the example above, we defined custom IntConverter class. And showed how to format 'Id' JSON property with leading zeros.

7.3.2. Configuration Approach

This model is applicable to both dynamic and POCO entity object. This gives freedom to attach the converters to each property at runtime. This takes precedence over the declarative converters on POCO classes.

Listing 7.3.2.1 Specifying TypeConverters

ChoJSONRecordConfiguration config = new ChoJSONRecordConfiguration();

ChoJSONRecordFieldConfiguration idConfig = new ChoJSONRecordFieldConfiguration("Id");
idConfig.AddConverter(new IntConverter());
config.JSONRecordFieldConfigurations.Add(idConfig);

config.JSONRecordFieldConfigurations.Add(new ChoJSONRecordFieldConfiguration("Name"));
config.JSONRecordFieldConfigurations.Add(new ChoJSONRecordFieldConfiguration("Name1"));

In the above, we construct and attach the IntConverter to 'Id' field using AddConverter helper method in ChoJSONRecordFieldConfiguration object.

Likewise, if you want to remove any converter from it, you can use RemoveConverter on ChoJSONRecordFieldConfiguration object.

7.3.3. Custom Value Converter Approach

This approach allows to attach value converter to each JSON member using Fluent API. This is a quick way to handle any odd conversion process and avoid creating value converter class.

Listing 7.3.3.1 POCO Class

public class EmployeeRec
{
    [ChoJSONRecordField]
    public int Id { get; set; }
    [ChoJSONRecordField(2, FieldName ="Name", QuoteField = true)]
    [Required]
    [DefaultValue("ZZZ")]
    public string Name { get; set; }
}

With the Fluent API, the sample below shows how to attach value converter to Id column.

Listing 7.3.3.2 Attaching Value Converter

using (var dr = new ChoJSONWriter<EmployeeRec>(@"Test.json")
    .WithField(c => c.Id, valueConverter: (v) => 
              ((int)value).ToString("C3", CultureInfo.CurrentCulture))
    )
{
    Console.WriteLine(rec);
}

7.4. Validations

JSONWriter leverages both System.ComponentModel.DataAnnotations and Validation Block validation attributes to specify validation rules for individual fields of POCO entity. Refer to the MSDN site for a list of available DataAnnotations validation attributes.

Listing 7.4.1 Using Validation Attributes in POCO Entity

[ChoJSONRecordObject]
public partial class EmployeeRec
{
    [ChoJSONRecordField(1, FieldName = "id")]
    [ChoTypeConverter(typeof(IntConverter))]
    [Range(1, int.MaxValue, ErrorMessage = "Id must be > 0.")]
    [ChoFallbackValue(1)]
    public int Id { get; set; }
 
    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    [ChoFallbackValue("XXX")]
    public string Name { get; set; }
}

In the example above, I have used Range validation attribute for Id property. Required validation attribute to Name property. JSONWriter performs validation on them before saving the data to file when Configuration.ObjectValidationMode is set to ChoObjectValidationMode.MemberLevel or ChoObjectValidationMode.ObjectLevel.

In some cases, you may want to take control and perform manual self validation within the POCO entity class. This can be achieved by inheriting POCO object from IChoValidatable interface.

Listing 7.4.2 Manual Validation on POCO Entity

[ChoJSONRecordObject]
public partial class EmployeeRec : IChoValidatable
{
    [ChoJSONRecordField]
    [ChoTypeConverter(typeof(IntConverter))]
    [Range(1, int.MaxValue, ErrorMessage = "Id must be > 0.")]
    [ChoFallbackValue(1)]
    public int Id { get; set; }
 
    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    [ChoFallbackValue("XXX")]
    public string Name { get; set; }
 
    public bool TryValidate
    (object target, ICollection<ValidationResult> validationResults)
    {
        return true;
    }
 
    public bool TryValidateFor(object target, string memberName, 
                ICollection<ValidationResult> validationResults)
    {
        return true;
    }
}

The sample above shows how to implement custom self-validation in POCO object.

IChoValidatable interface exposes the below methods:

TryValidate - Validate entire object, return true if all validation passed. Otherwise, return false.
TryValidateFor - Validate specific property of the object, return true if all validation passed. Otherwise, return false.

7.5. ChoIgnoreMember

If you want to ignore a POCO class member from JSON parsing in OptOut mode, decorate them with ChoIgnoreMemberAttribute. The sample below shows Title member is ignored from JSON loading process.

Listing 7.5.1 Ignore a Member

public class EmployeeRec
{
    public int Id { get; set; }
    public string Name { get; set; }
    [ChoIgnoreMember]
    public string Title { get; set; }
}

7.6. StringLength

In OptOut mode, you can specify the size of the JSON column by using System.ComponentModel.DataAnnotations.StringLengthAttribute.

Listing 7.6.1 Specifying Size of JSON Member

public class EmployeeRec
{
    public int Id { get; set; }
    [StringLength(25)]
    public string Name { get; set; }
    [ChoIgnoreMember]
    public string Title { get; set; }
}

7.6. Display

In OptOut mode, you can specify the name of JSON column mapped to member using System.ComponentModel.DataAnnotations.DisplayAttribute.

Listing 7.6.1 Specifying Name of JSON Column

public class EmployeeRec
{
    public int Id { get; set; }
    [Display(Name="FullName")]
    [StringLength(25)]
    public string Name { get; set; }
    [ChoIgnoreMember]
    public string Title { get; set; }
}

7.7. DisplayName

In OptOut mode, you can specify the name of JSON column mapped to member using System.ComponentModel.DataAnnotations.DisplayNameAttribute.

Listing 7.7.1 Specifying Name of JSON Column

public class EmployeeRec
{
    public int Id { get; set; }
    [Display(Name="FullName")]
    [StringLength(25)]
    public string Name { get; set; }
    [ChoIgnoreMember]
    public string Title { get; set; }
}

8. Callback Mechanism

JSONWriter offers industry standard JSON data file generation out of the box to handle most of the needs. If the generation process is not handling any of your needs, you can use the callback mechanism offered by JSONWriter to handle such situations. In order to participate in the callback mechanism, you can use one of the following models:

Using event handlers exposed by JSONWriter via IChoWriter interface
Inheriting POCO entity object from IChoNotifyRecordWrite / IChoNotifyFileWrite / IChoNotifyRecordFieldWrite interfaces
Inheriting DataAnnotation's MetadataType type object by IChoNotifyRecordWrite / IChoNotifyFileWrite / IChoNotifyRecordFieldWrite interfaces.

In order to participate in the callback mechanism, either POCO entity object or DataAnnotation's MetadataType type object must be inherited by IChoNotifyRecordWrite interface.

Tip: Any exceptions raised out of these interface methods will be ignored.

IChoRecorder exposes the below methods:

BeginWrite - Invoked at the beginning of the JSON file write
EndWrite - Invoked at the end of the JSON file write
BeforeRecordWrite - Raised before the JSON record write
AfterRecordWrite - Raised after JSON record write
RecordWriteError - Raised when JSON record errors out while writing
BeforeRecordFieldWrite - Raised before JSON column value write
AfterRecordFieldWrite - Raised after JSON column value write
RecordFieldWriteError - Raised when JSON column value errors out while writing

IChoNotifyRecordWrite exposes the below methods:

BeforeRecordWrite - Raised before the JSON record write
AfterRecordWrite - Raised after JSON record write
RecordWriteError - Raised when JSON record write errors out

IChoNotifyFileWrite exposes the below methods:

BeginWrite - Invoked at the beginning of the JSON file write
EndWrite - Invoked at the end of the JSON file write

IChoNotifyRecordFieldWrite exposes the below methods:

BeforeRecordFieldWrite - Raised before JSON column value write
AfterRecordFieldWrite - Raised after JSON column value write
RecordFieldWriteError - Raised when JSON column value write errors out

IChoNotifyFileHeaderArrange exposes the below methods:

FileHeaderArrange - Raised before JSON file header is written to file, an opportunity to rearrange the JSON columns

IChoNotifyFileHeaderWrite exposes the below methods:

FileHeaderWrite - Raised before JSON file header is written to file, an opportunity to customize the header.

8.1 Using JSONWriter Events

This is more direct and the simplest way to subscribe to the callback events and handle your odd situations in parsing JSON files. The downside is that code can't be reusable as you do by implementing IChoNotifyRecordRead with POCO record object.

The sample below shows how to use the BeforeRecordLoad callback method to skip lines stating with '%' characters.

Listing 8.1.1 Using JSONWriter Callback Events

static void IgnoreLineTest()
{
    using (var parser = new ChoJSONWriter("emp.json"))
    { 
        parser.BeforeRecordWrite += (o, e) =>
        {
            if (e.Source != null)
            {
                e.Skip = ((JObject)e.Source).Contains("name1");
            }
        };
        
        parser.Write(rec);
    }
}

Likewise, you can use other callback methods as well with JSONWriter.

8.2 Implementing IChoNotifyRecordWrite Interface

The sample below shows how to implement IChoNotifyRecordWrite interface to direct POCO class.

Listing 8.2.1 Direct POCO Callback Mechanism Implementation

[ChoJSONRecordObject]
public partial class EmployeeRec : IChoNotifyrRecordWrite
{
    [ChoJSONRecordField]
    [ChoTypeConverter(typeof(IntConverter))]
    [Range(1, int.MaxValue, ErrorMessage = "Id must be > 0.")]
    [ChoFallbackValue(1)]
    public int Id { get; set; }
    
    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    [ChoFallbackValue("XXX")]
    public string Name { get; set; }

    public bool AfterRecordWrite(object target, int index, object source)
    {
        throw new NotImplementedException();
    }

    public bool BeforeRecordWrite(object target, int index, ref object source)
    {
        throw new NotImplementedException();
    }

    public bool RecordWriteError(object target, int index, object source, Exception ex)
    {
        throw new NotImplementedException();
    }
}

The sample below shows how to attach Metadata class to POCO class by using MetadataTypeAttribute on it.

Listing 8.2.2 MetaDataType Based Callback Mechanism Implementation

[ChoJSONRecordObject]
public class EmployeeRecMeta : IChoNotifyRecordWrite
{
    [ChoJSONRecordField]
    [ChoTypeConverter(typeof(IntConverter))]
    [Range(1, int.MaxValue, ErrorMessage = "Id must be > 0.")]
    [ChoFallbackValue(1)]
    public int Id { get; set; }

    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    [ChoFallbackValue("XXX")]
    public string Name { get; set; }

    public bool AfterRecordWrite(object target, int index, object source)
    {
        throw new NotImplementedException();
    }

    public bool BeforeRecordWrite(object target, int index, ref object source)
    {
        throw new NotImplementedException();
    }

    public bool RecordWriteError(object target, int index, object source, Exception ex)
    {
        throw new NotImplementedException();
    }
} 

[MetadataType(typeof(EmployeeRecMeta))]
public partial class EmployeeRec
{
    public int Id { get; set; }
    public string Name { get; set; }
}

The sample below shows how to attach Metadata class for sealed or third party POCO class by using ChoMetadataRefTypeAttribute on it.

Listing 8.2.2 MetaDataType Based Callback Mechanism Implementation

[ChoMetadataRefType(typeof(EmployeeRec))]
[ChoJSONRecordObject]
public class EmployeeRecMeta : IChoNotifyRecordWrite
{
    [ChoJSONRecordField]
    [ChoTypeConverter(typeof(IntConverter))]
    [Range(1, int.MaxValue, ErrorMessage = "Id must be > 0.")]
    [ChoFallbackValue(1)]
    public int Id { get; set; }

    [ChoJSONRecordField]
    [Required]
    [DefaultValue("ZZZ")]
    [ChoFallbackValue("XXX")]
    public string Name { get; set; }

    public bool AfterRecordWrite(object target, int index, object source)
    {
        throw new NotImplementedException();
    }

    public bool BeforeRecordWrite(object target, int index, ref object source)
    {
        throw new NotImplementedException();
    }

    public bool RecordWriteError(object target, int index, object source, Exception ex)
    {
        throw new NotImplementedException();
    }
} 

public partial class EmployeeRec
{
    public int Id { get; set; }
    public string Name { get; set; }
}

8.3 BeginWrite

This callback is invoked once at the beginning of the JSON file write. source is the JSON file stream object. In here, you have a chance to inspect the stream, return true to continue the JSON generation. Return false to stop the generation.

Listing 8.3.1 BeginWrite Callback Sample

public bool BeginWrite(object source)
{
    StreamReader sr = source as StreamReader;
    return true;
}

8.4 EndWrite

This callback is invoked once at the end of the JSON file generation. source is the JSON file stream object. In here, you have a chance to inspect the stream, do any post steps to be performed on the stream.

Listing 8.4.1 EndWrite Callback Sample

public void EndWrite(object source)
{
    StreamReader sr = source as StreamReader;
}

8.5 BeforeRecordWrite

This callback invoked before each POCO record object is written to JSON file. target is the instance of the POCO record object. index is the line index in the file. source is the JSON record line. In here, you have a chance to inspect the POCO object, and generate the JSON record line if needed.

Tip: If you want to skip the record from writing, set the source to null.