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MarshalAsAttribute.SizeParamIndex Field

Indicates the zero-based parameter that contains the count of array elements, similar to size_is in COM.

Namespace:  System.Runtime.InteropServices
Assembly:  mscorlib (in mscorlib.dll)

public short SizeParamIndex

The SizeParamIndex field supports managed-to-unmanaged and unmanaged-to-managed calls. It does not have any effect on managed code that calls COM objects.

Depending on the managed type and the attributes applied to it, the array can be passed as a safe array or C-style array.

When arrays are passed as C-style arrays, the marshaler cannot determine the size of the array. Therefore, to pass an managed array to an unmanaged function or method, you must provide two arguments:

  • The array, defined by reference or value.

  • The array size, defined by reference or value.

The zero-based index of the array size parameter is defined by using the SizeParamIndex field.

If you specify both SizeParamIndex and MarshalAsAttribute.SizeConst with a UnmanagedType.LPArray field, the sum of the fields' values produces a size total.

For more information, see Default Marshaling for Arrays.

using System.Runtime.InteropServices;
using SomeNamespace;

namespace SomeNamespace
    // Force the layout of your fields to the C style struct layout.
    // Without this, the .NET Framework will reorder your fields.
    public struct Vertex
    	float	x;
	float	y;
    	float	z;

    class SomeClass
        // Add [In] or [In, Out] attributes as approppriate.
        // Marshal as a C style array of Vertex, where the second (SizeParamIndex is zero-based)
        //  parameter (size) contains the count of array elements.
        [DllImport ("SomeDll.dll")]
        public static extern void SomeUnsafeMethod(
                                      [MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)] Vertex[] data,
                                      long size );

        public void SomeMethod()
            Vertex[] verts = new Vertex[3];
            SomeUnsafeMethod( verts, verts.Length );


class Test
	public static void Main()
        SomeClass AClass = new SomeClass();


.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.