C# DllImport Guide: Usage Explained

DllImport is a feature in C# that allows for the interaction between C# code and external unmanaged code.

The main purpose of DllImport is to declare a method that is implemented in unmanaged code and can be invoked in C#. When using DllImport, you need to provide the name of the external function and the path to the dynamic link library (DLL) containing that function.

The steps to use DllImport are as follows:

1. Declare a static extern method in C# with a method name and parameters that match those in the unmanaged code.
2. Decorate this method with the DllImport attribute and specify the name of the external function along with the path to the DLL.
3. Call the method in C# code.

Here is an example of using DllImport:

using System;
using System.Runtime.InteropServices;

public class Program
{
    // 声明一个外部函数，函数在非托管代码中实现
    [DllImport("user32.dll")]
    public static extern int MessageBox(IntPtr hWnd, string text, string caption, uint type);

    public static void Main()
    {
        // 调用外部函数
        MessageBox(IntPtr.Zero, "Hello World!", "MessageBox", 0);
    }
}

In the above example, the DllImport attribute is used to declare an external function MessageBox, which is implemented in the user32.dll dynamic link library on Windows. The function is then called in the Main method to display a message box.

It is important to ensure the correctness and availability of unmanaged code when using DllImport, otherwise it may result in runtime errors.