How to use [DllImport("")] in C#?

Question

I found a lot of questions about it, but no one explains how I can use this.

I have this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using Microsoft.FSharp.Linq.RuntimeHelpers;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.IO;

public class WindowHandling
{
    public void ActivateTargetApplication(string processName, List<string> barcodesList)
    {
        [DllImport("User32.dll")]
        public static extern int SetForegroundWindow(IntPtr point);
        Process p = Process.Start("notepad++.exe");
        p.WaitForInputIdle();
        IntPtr h = p.MainWindowHandle;
        SetForegroundWindow(h);
        SendKeys.SendWait("k");
        IntPtr processFoundWindow = p.MainWindowHandle;
    }
}

Can someone help me to understand why it gives me an error on the DllImport line and on the public static line?

Does anyone have an idea, what can I do? Thank you.

There are multiple issues in addition to what @vcsjones has mentioned. Do you have User32.dll where you are using this. And also check the place where you have written the [DllImport] statement.Its wrong place. — PM., Oct 18 '13 at 13:26
@PM. User32.dll is a pretty standard Win32 DLL that will always be in SysWow64 or System32, so there shouldn't be an issue with finding it. It might find the *wrong* one if there is another DLL with the same name that is "closer" in the search sequence, but that would bring disaster to most programs. — vcsjones, Oct 18 '13 at 13:27

vcsjones · Accepted Answer · 2018-01-22T13:51:12.097

95

You can't declare an extern local method inside of a method, or any other method with an attribute. Move your DLL import into the class:

using System.Runtime.InteropServices;


public class WindowHandling
{
    [DllImport("User32.dll")]
    public static extern int SetForegroundWindow(IntPtr point);

    public void ActivateTargetApplication(string processName, List<string> barcodesList)
    {
        Process p = Process.Start("notepad++.exe");
        p.WaitForInputIdle();
        IntPtr h = p.MainWindowHandle;
        SetForegroundWindow(h);
        SendKeys.SendWait("k");
        IntPtr processFoundWindow = p.MainWindowHandle;
    }
}

edited Jan 22 '18 at 13:51

answered Oct 18 '13 at 13:24

vcsjones

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35

You must also be using `System.Runtime.InteropServices` in Microsoft Visual Studio – Cullub Sep 23 '14 at 12:41
3

You must also be using `System.Diagnostics` in order to use the Process object – Andrew Jay Feb 02 '18 at 15:41

Coconut9 · Answer 2 · 2021-03-27T16:25:17.800

Starting with C# 9, your syntax would be valid if you remove the public keyword from the SetForegroundWindow deceleration:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using Microsoft.FSharp.Linq.RuntimeHelpers;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.IO;

public class WindowHandling
{
    public void ActivateTargetApplication(string processName, List<string> barcodesList)
    {
        [DllImport("User32.dll")]
        static extern int SetForegroundWindow(IntPtr point);
        Process p = Process.Start("notepad++.exe");
        p.WaitForInputIdle();
        IntPtr h = p.MainWindowHandle;
        SetForegroundWindow(h);
        SendKeys.SendWait("k");
        IntPtr processFoundWindow = p.MainWindowHandle;
    }
}

In C# 9 local functions can have attributes, see here

How to use [DllImport("")] in C#?

2 Answers2

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