What are Anonymous types ? Anonymous types are the new concept in C#3.0 which allow to create new types without defining them. Why are Anonymous types introduced in C#? Anonymous types have been introduced to support one of the most useful features called LINQ. It's most useful when you are querying collection of object using LINQ query and you want to return only a few of its properties. How to define Anonymous types ? One can define Anonymous types easily by using new keyword of C#. Here is the example of it:

var pranay = new { id=1, Name= "pranay rana" };
var krunal = new { id=1, Name= "krunal mevada" };

LINQ query example

Querying the object collection having property Id, Name and Age but you just want to get only Id and Name after querying data, then code is like:

var user = from user in Users
                select new { user.Name, user.Id}

Here as you see select new i.e second line of code generates anonymous type. Consider the below code to understand Anonymous type in more detail. To define array of Anonymous type:

var persons = new[] { 
               new { id=1, Name= "pranay rana" },
               new { id=2, Name= "krunal mevada" },
               new { id=3, Name= "hemang vyas" } 
             };

          foreach (var person in persons)
            {
                Console.WriteLine("Person : " + person.id);
                Console.WriteLine("Person : " + person.Name);
            }

After compiling code, when you see your DLL or EXE file in ILDASM, it shows code generated by the compiler.

Line of code defines the Anonymous type which is generated by compiler

locals init ([0] class '<>f__AnonymousType0`2'<int32,string>[] persons,
           [1] class '<>f__AnonymousType0`2'<int32,string> person,
           [2] class '<>f__AnonymousType0`2'<int32,string>[] CS$0$0000,
           [3] class '<>f__AnonymousType0`2'<int32,string>[] CS$6$0001,
           [4] int32 CS$7$0002,
           [5] bool CS$4$0003)

Array of the type is defined as:

IL_0000:  nop
  IL_0001:  ldc.i4.3
  IL_0002:  newarr     class '<>f__AnonymousType0`2'<int32,string>

Create instance of Anonymous type:

IL_0015:  stelem.ref
  IL_0016:  ldloc.2
  IL_0017:  ldc.i4.1
  IL_0018:  ldc.i4.2
  IL_0019:  ldstr      "krunal mevada"
  IL_001e:  newobj     instance void class '<>f__AnonymousType0`2'<int32,string>::.ctor(!0,
                                                                                        !1)

Whole source code:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       136 (0x88)
  .maxstack  5
  .locals init ([0] class '<>f__AnonymousType0`2'<int32,string>[] persons,
           [1] class '<>f__AnonymousType0`2'<int32,string> person,
           [2] class '<>f__AnonymousType0`2'<int32,string>[] CS$0$0000,
           [3] class '<>f__AnonymousType0`2'<int32,string>[] CS$6$0001,
           [4] int32 CS$7$0002,
           [5] bool CS$4$0003)
  IL_0000:  nop
  IL_0001:  ldc.i4.3
  IL_0002:  newarr     class '<>f__AnonymousType0`2'<int32,string>
  IL_0007:  stloc.2
  IL_0008:  ldloc.2
  IL_0009:  ldc.i4.0
  IL_000a:  ldc.i4.1
  IL_000b:  ldstr      "pranay rana"
  IL_0010:  newobj     instance void class '<>f__AnonymousType0`2'<int32,string>::.ctor(!0,
                                                                                        !1)
  IL_0015:  stelem.ref
  IL_0016:  ldloc.2
  IL_0017:  ldc.i4.1
  IL_0018:  ldc.i4.2
  IL_0019:  ldstr      "krunal mevada"
  IL_001e:  newobj     instance void class '<>f__AnonymousType0`2'<int32,string>::.ctor(!0,
                                                                                        !1)
  IL_0023:  stelem.ref
  IL_0024:  ldloc.2
  IL_0025:  ldc.i4.2
  IL_0026:  ldc.i4.3
  IL_0027:  ldstr      "hemang vyas"
  IL_002c:  newobj     instance void class '<>f__AnonymousType0`2'<int32,string>::.ctor(!0,
                                                                                        !1)
  IL_0031:  stelem.ref
  IL_0032:  ldloc.2
  IL_0033:  stloc.0
  IL_0034:  nop
  IL_0035:  ldloc.0
  IL_0036:  stloc.3
  IL_0037:  ldc.i4.0
  IL_0038:  stloc.s    CS$7$0002
  IL_003a:  br.s       IL_007a
  IL_003c:  ldloc.3
  IL_003d:  ldloc.s    CS$7$0002
  IL_003f:  ldelem.ref
  IL_0040:  stloc.1
  IL_0041:  nop
  IL_0042:  ldstr      "Person : "
  IL_0047:  ldloc.1
  IL_0048:  callvirt   instance !0 class '<>f__AnonymousType0`2'<int32,string>::get_id()
  IL_004d:  box        [mscorlib]System.Int32
  IL_0052:  call       string [mscorlib]System.String::Concat(object,
                                                              object)
  IL_0057:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_005c:  nop
  IL_005d:  ldstr      "Person : "
  IL_0062:  ldloc.1
  IL_0063:  callvirt   instance !1 class '<>f__AnonymousType0`2'<int32,string>::get_Name()
  IL_0068:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_006d:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0072:  nop
  IL_0073:  nop
  IL_0074:  ldloc.s    CS$7$0002
  IL_0076:  ldc.i4.1
  IL_0077:  add
  IL_0078:  stloc.s    CS$7$0002
  IL_007a:  ldloc.s    CS$7$0002
  IL_007c:  ldloc.3
  IL_007d:  ldlen
  IL_007e:  conv.i4
  IL_007f:  clt
  IL_0081:  stloc.s    CS$4$0003
  IL_0083:  ldloc.s    CS$4$0003
  IL_0085:  brtrue.s   IL_003c
  IL_0087:  ret
} // end of method Program::Main

Quick facts about Anonymous type

Anonymous types are reference types derived form system.objects.
Properties of the Anonymous type are read only.
If two Anonymous types have the same properties and same order, then the compiler treats it as the same type, but if both are in one assembly.
Anonymous type has method scope. If you want to return Anonymous type from the method, then you have to convert it in object type. But is not good practice.

Find more at : http://pranayamr.blogspot.com/