MarshalAsAttribute Class


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Indicates how to marshal the data between managed and unmanaged code.

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


[AttributeUsageAttribute(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.ReturnValue, 
	Inherited = false)]
public sealed class MarshalAsAttribute : Attribute


Initializes a new instance of the MarshalAsAttribute class with the specified UnmanagedType value.


Initializes a new instance of the MarshalAsAttribute class with the specified UnmanagedType enumeration member.


When implemented in a derived class, gets a unique identifier for this Attribute.(Inherited from Attribute.)


Gets the UnmanagedType value the data is to be marshaled as.


This API supports the product infrastructure and is not intended to be used directly from your code. Returns a value that indicates whether this instance is equal to a specified object.(Inherited from Attribute.)


Returns the hash code for this instance.(Inherited from Attribute.)


Gets the Type of the current instance.(Inherited from Object.)


When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class.(Inherited from Attribute.)


When overridden in a derived class, returns a value that indicates whether this instance equals a specified object.(Inherited from Attribute.)


Returns a string that represents the current object.(Inherited from Object.)


Specifies the element type of the unmanaged UnmanagedType.LPArray or UnmanagedType.ByValArray.


Specifies the parameter index of the unmanaged iid_is attribute used by COM.


Provides additional information to a custom marshaler.


Specifies the fully qualified name of a custom marshaler.


Implements MarshalAsAttribute.MarshalType as a type.


Indicates the element type of the UnmanagedType.SafeArray.


Indicates the user-defined element type of the UnmanagedType.SafeArray.


Indicates the number of elements in the fixed-length array or the number of characters (not bytes) in a string to import.


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

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetIDsOfNames(Guid, IntPtr, UInt32, UInt32, IntPtr)

Maps a set of names to a corresponding set of dispatch identifiers.(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetTypeInfo(UInt32, UInt32, IntPtr)

Retrieves the type information for an object, which can be used to get the type information for an interface.(Inherited from Attribute.)


Retrieves the number of type information interfaces that an object provides (either 0 or 1).(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.Invoke(UInt32, Guid, UInt32, Int16, IntPtr, IntPtr, IntPtr, IntPtr)

Provides access to properties and methods exposed by an object.(Inherited from Attribute.)

You can apply this attribute to parameters, fields, or return values.

This attribute is optional, as each data type has a default marshaling behavior. This attribute is only necessary when a given type can be marshaled to multiple types. For example, you can marshal a string to unmanaged code as either a LPStr, a LPWStr, a LPTStr, or a BStr. By default, the common language runtime marshals a string parameter as a BStr to COM methods. You can apply the MarshalAsAttribute attribute to an individual field or parameter to cause that particular string to be marshaled as a LPStr instead of a BStr. The Tlbexp.exe (Type Library Exporter) passes your marshaling preferences to the common language runtime.

Some parameters and return values have different default marshaling behavior when used with COM interop or platform invoke. By default, the runtime marshals a string parameter (and fields in a value type) as a LPStr to a platform invoke method or function. For additional information, see Default Marshaling Behavior.

In most cases, the attribute simply identifies the format of the unmanaged data using the UnmanagedType enumeration, as shown in the following C# signature:

public void SomeMethod([MarshalAs(UnmanagedType.LPStr)] String s)

Some UnmanagedType enumeration members require additional information. For example, additional information is needed when the UnmanagedType is LPArray. For a complete description of how to use this attribute with arrays, see Default Marshaling for Arrays.

The Tlbimp.exe (Type Library Importer) also applies this attribute to parameters, fields, and return values to indicate that the data type in the input type library is not the default type for the corresponding managed data type. Tlbimp.exe always applies the MarshalAsAttribute to String and Object types for clarity, regardless of the type specified in the input type library.


The MarshalAsAttribute does not support marshaling of generic types.

The following example applies the MarshalAsAttribute to a field, a method parameter, and a method return value in a managed type.

using System;
using System.Text;
using System.Runtime.InteropServices;

class Program

//Applied to a parameter.
  public void M1([MarshalAs(UnmanagedType.LPWStr)]String msg) {}

//Applied to a field within a class.
  class MsgText {
                public String msg = "Hello World";

//Applied to a return value.
[return: MarshalAs(UnmanagedType.LPWStr)]
    public String GetMessage()
        return "Hello World";

static void Main(string[] args)
    {  }

The following example applies the MarshalAsAttribute attribute to a property:

decimal _money;   

public decimal Money 
   [return: MarshalAs(UnmanagedType.Currency)]
   get { return this._money; }
   [param: MarshalAs(UnmanagedType.Currency)]
   set { this._money = value; }

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1

Any public static ( Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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