This attribute (and, in C#, the keyword "extern") specifies that a method definition is implemented externally (usually in a DLL). Apply this attribute to a method that has been declared (but not defined) to specify the DLL name and entry point in which the method can be found.

The attribute can be customized in a number of different ways to help control the binding against the external method. The CallingConvention value dictates how the parameters to the call (and return value coming back) should be sent to the function. CallingConvention.StdCall (used for calling into _stdcall-declared functions, which is most of the Win32 API set) and CallingConvention.Cdecl (used for calling functions declared directly from C or C++) are the two most common values. The CharSet value indicates which character set parameters to the call are expected to be, either two-byte Unicode or one-byte ANSI. EntryPoint indicates the name of the exported function from the DLL to bind to (normally this is guessed from the name of the .NET-declared method), and ExactSpelling indicates whether the .NET compiler should attempt to "best match" a declared DllImport method against a possible set of exported functions. The PreserveSig value indicates how .NET should treat [out]-declared and [retval]-declared parameters. By default, the .NET compilers ignore the HRESULT return value on IDL-declared methods and use the [retval]-declared parameter as the return value; setting PreserveSig to true turns this off. BestFitMapping is used to define whether the CLR should try to "best fit" Unicode characters to ANSI equivalents, and is turned on by default. ThrowOnUnmappableChar indicates whether an exception should be thrown when the interop marshaler attempts to convert an unmappable character, and is turned off by default. Finally, because many Win32 APIs use the GetLastError API call to note the exact reason a call fails, the SetLastError value indicates whether the caller should use that API to discover the reason for failures.

public sealed class DllImportAttribute : Attribute {
// Public Constructors
   public DllImportAttribute(string dllName);
// Public Instance Fields
   public bool BestFitMapping;
   public CallingConvention CallingConvention;
   public CharSet CharSet;
   public string EntryPoint;
   public bool ExactSpelling;
   public bool PreserveSig;
   public bool SetLastError;
   public bool ThrowOnUnmappableChar;
// Public Instance Properties
   public string Value{get; }
}

Hierarchy

System.Object System.Attribute DllImportAttribute

Valid On

Method