Make method with string[] with optional default values

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Hi Folks,

How to have a method that has a string[] array value with default optional values.

In gridviews most database have the fields ID and DateModified setup for GridView.DataKeyNames.
I have 1 application that have ID and date_modified als columns for GridView.DataKeyNames

In my method I want to do
void MyMethod(string[] datakeyNames = new string[] { "Id", "DateModified"})

How can I set-up default values for a string array in a method?

What I have tried:

All I get from C# compiler is the message:
Default parameter value for 'datakeyNames' must be a compile-time constant

Posted 2-Jul-19 1:37am

Herman<T>.Instance

Updated 2-Jul-19 16:41pm

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phil.o · Accepted Answer · 2019-07-02T01:42:00

Solution 1

You cannot define a default value for an array as a parameter, because this way the default value has to be a compile-time constant.
You can however test for a null value, and assign the default value accordingly:

C#

void MyMethod(string[] datakeyNames = null)
{
   datakeyNames = datakeyNames ?? new string[] { "Id", "DateModified" };
   // ...
}

Posted 2-Jul-19 1:42am

phil.o

Updated 2-Jul-19 1:43am

v2

Comments

TheRealSteveJudge 2-Jul-19 7:43am

Elegant workaround! 5*

phil.o 2-Jul-19 8:11am

Thank you :)

Maciej Los 2-Jul-19 8:27am

5ed!

#realJSOP · Accepted Answer · 2019-07-02T01:47:00

You can't do it with default parameters (for an array), but you can do it with an overload:

C#

public bool MyMethod()
{
    string[] value = new string[] { "Id", "DataModified" };
    return MyMethod(value);
}

public bool MyMethod(string[] value)
{
    // do something with the string array
    return false;
}

This way, you can call either overload and be okay. If you combine the two answers, you could do something like this:

C#

protected string[] DefaultValues()
{
    return new string[] { "Id", "DataModified" };
}

public bool MyMethod()
{
    return MyMethod(this.DefaultValues());
}

public bool MyMethod(string[] value = null)
{
    value = value ?? this.DefaultValues();
    // do something with the string array
    return false;
}

That way, you cover all of your bases, and still only have to change the default values in one location.

BillWoodruff · Accepted Answer · 2019-07-02T16:42:00

This is an alternative pattern I sometimes use for handling a mix of optional and default parameters:

private string[] defaultData = new []{"Id", "DataModified"};

// requires C# >= 4.00
// requires Linq
public bool myMethod(bool ensureDefaultDataIsUsed = true, params string[] data)
{
    if (ensureDefaultDataIsUsed)
    {
        data = data.Concat(defaultData).Distinct().ToArray();
    }

    return true;
}

Consider these usage examples:

// no parameters: 'defaultData is used
bool m0 = myMethod();

// single parameter: 'defaultData combines with parameter
bool m1 = myMethod(data: "hello");

// multiple parameters: 'defaultData combines with parameters
bool m2 = myMethod(true, "hello", "goodbye");

// multiple parameters: 'defaultData is not used
bool m3 = myMethod(false, "hello", "goodbye");

// will not compile
bool m4 = myMethod("hello");

What I like about this:

1 keeping default parameters in a variable makes it easy to change what the defaults are

2 all the options are in one method

3 control over whether defaults are used, ignored, or combined with user supplied data

What some might consider the downside: example #m4 illustrates that using an explicitly named parameter for 'ensureDefaultDataIsUsed is required to avoid a clash between a positional and an optional value.

Make method with string[] with optional default values

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