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TypeBuilder.DefineUninitializedData Method

Defines an uninitialized data field in the .sdata section of the portable executable (PE) file.

Namespace:  System.Reflection.Emit
Assembly:  mscorlib (in mscorlib.dll)

public FieldBuilder DefineUninitializedData(
	string name,
	int size,
	FieldAttributes attributes


Type: System.String
The name used to refer to the data. name cannot contain embedded nulls.
Type: System.Int32
The size of the data field.
Type: System.Reflection.FieldAttributes
The attributes for the field.

Return Value

Type: System.Reflection.Emit.FieldBuilder
A field to reference the data.


Length of name is zero.


size is less than or equal to zero, or greater than or equal to 0x003f0000.


name is null.


The type was previously created using CreateType.

The field that you create with this method will be static, even if you do not include FieldAttributes.Static in the attributes parameter.

The following code sample demonstrates the use of DefineUninitializedData to create an uninitialized data field in a dynamic type:

using System;
using System.Threading;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.InteropServices;
using System.Security.Permissions;

public sealed class Example
   [PermissionSetAttribute(SecurityAction.Demand, Name="FullTrust")]
   public static void Main()
      Type myHelloWorldType = CreateCallee(Thread.GetDomain());
      object myHelloWorldInstance =
      FieldInfo myGreetingFieldInfo =
      object oval = Activator.CreateInstance(myGreetingFieldInfo.FieldType);
      IntPtr myIntPtr = Marshal.AllocHGlobal(4);
      Random rand = new Random();
      int iTempSeed = rand.Next();
      byte[] bINITBYTE = GetRandBytes( iTempSeed, 4);
      IntPtr intptrTemp = myIntPtr;
      for ( int j = 0; j < 4; j++ )
         Marshal.WriteByte( myIntPtr, bINITBYTE[j]);
         myIntPtr = (IntPtr)((int)myIntPtr + 1);
      myIntPtr = intptrTemp;
      Object oValNew = Marshal.PtrToStructure( myIntPtr, myGreetingFieldInfo.FieldType);
      Marshal.FreeHGlobal( myIntPtr );

      myIntPtr = Marshal.AllocHGlobal(4);
      object myObj = myGreetingFieldInfo.GetValue(myHelloWorldInstance);
      Marshal.StructureToPtr(myObj, myIntPtr, true);
      intptrTemp = myIntPtr;
      Console.WriteLine("The value of 'MyGreeting' field : ");
      for ( int j = 0; j < 4; j++ )
         Marshal.WriteByte( myIntPtr, bINITBYTE[j]);
         myIntPtr = (IntPtr)((int)myIntPtr + 1);

   private static byte[] GetRandBytes( int iRandSeed, int iSize )
      byte[] barr = new byte[iSize];
      Random randTemp = new Random( iRandSeed );
      randTemp.NextBytes( barr );
      return barr;

   // Create the callee transient dynamic assembly.
   private static Type CreateCallee(AppDomain myDomain)
      // Create a simple name for the callee assembly.
      AssemblyName myAssemblyName = new AssemblyName();
      myAssemblyName.Name = "EmittedClass";

      // Create the callee dynamic assembly.
      AssemblyBuilder myAssembly =

      // Create a dynamic module in the callee assembly.
      ModuleBuilder myModule =

      // Define a public class named "MyHelloWorld"
      TypeBuilder myHelloWorldType =
         myModule.DefineType("MyHelloWorld", TypeAttributes.Public);

      // Define a 'MyGreeting' field and initialize it.
      FieldBuilder myFieldBuilder =

      // Create the 'MyHelloWorld' class.

   private Example() {}

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.