.NET: Call an External Function Using DllImport in C#

Learn the difference between "cdecl" and "stdcall" with assembly.

Declare a callback function for a .DLL file written in C/C++.

When writing external functions for use by .NET, it may help to know which calling convention to use. Generally, use CallingConvention.StdCall which can be declared in Microsoft's Visual Studio C++ compiler by simply adding a __stdcall between the function return type and the function name. Also, make sure to declare the function an extern "C" function by wrapping it and others in a extern "C" { } block of code. This prevents the C++ compiler from mangling the name of the function up.

The IntPtr type is an interesting one. Use it for pointers to objects in memory, not primitive data types. A pointer to a pointer would be declared out IntPtr. There is an example of this below.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Dynamic;
using System.Runtime.InteropServices;
using System.Text;

namespace Code
{
	class FunctionImports
	{
		// NOTE: STDCALL: The callee cleans the stack. The majority of Window API functions use this calling convention.
		[DllImport("kernel32", CallingConvention = CallingConvention.StdCall)]
		private static extern uint SetThreadExecutionState(uint esFlags);


		// NOTE: CDECL: The caller cleans the stack.This enables calling functions with variable arguments because the caller knows
		// how many were passed.This makes it appropriate to use methods that accept a variable number of parameters, like printf() does.
		[DllImport("prowaretech.dll", CallingConvention = CallingConvention.Cdecl)]
		private static extern uint SomeFunc(double[] floating_points);


		// NOTE: The name of the function as exported by the library is "UglyFuncName" and this
		// is a useful way to import a function while giving it a more descriptive name.
		[DllImport("prowaretech.dll", EntryPoint = "UglyFuncName", CallingConvention = CallingConvention.StdCall)]
		private static extern IntPtr PrettyFuncName(out IntPtr ppVar); // NOTE: a pointer to a pointer


		// NOTE: FASTCALL: FastCall is not supported! Use at one's own discretion!
		[DllImport("prowaretech.dll", CallingConvention = CallingConvention.FastCall)]
		private static extern void BasicFunc(string str);


		// NOTE: THISCALL: The first parameter is the "this" pointer and is stored in the ECX register. Other parameters
		// are pushed in the stack. This calling convention is used to call methods on classes from an unmanaged DLL.
		[DllImport("prowaretech.dll", CallingConvention = CallingConvention.ThisCall)]
		private static extern void BasicFuncOfAnObject(uint[] uint_array);
	}
}

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