Specify the search path for DllImport in .NET

Question

Is there a way to specify the paths to be searched for a given assembly that is imported with DllImport?

[DllImport("MyDll.dll")]
static extern void Func();

This will search for the dll in the app dir and in the PATH environment variable. But at times the dll will be placed elsewhere. Can this information be specified in app.config or manifest file to avoid dynamic loading and dynamic invocation?

score 70 · Accepted Answer · edited Jun 21 '19 at 21:29

70

Call SetDllDirectory with your additional DLL paths before you call into the imported function for the first time.

P/Invoke signature:

[DllImport("kernel32.dll", SetLastError = true)]
static extern bool SetDllDirectory(string lpPathName);

To set more than one additional DLL search path, modify the PATH environment variable, e.g.:

static void AddEnvironmentPaths(string[] paths)
{
    string path = Environment.GetEnvironmentVariable("PATH") ?? string.Empty;
    path += ";" + string.Join(";", paths);

    Environment.SetEnvironmentVariable("PATH", path);
}

There's more info about the DLL search order here on MSDN.

Updated 2013/07/30:

Updated version of the above using Path.PathSeparator:

static void AddEnvironmentPaths(IEnumerable<string> paths)
{
    var path = new[] { Environment.GetEnvironmentVariable("PATH") ?? string.Empty };

    string newPath = string.Join(Path.PathSeparator.ToString(), path.Concat(paths));

    Environment.SetEnvironmentVariable("PATH", newPath);
}

edited Jun 21 '19 at 21:29

Jon

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answered May 19 '10 at 10:39

Chris Schmich

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Thanks, works great after invoking SetDllDirectory from static constructor. – Stefan May 19 '10 at 11:09
AFAIK, unfortunatelly this does not solve the issue of multiple additional search directories. – Stefan May 19 '10 at 12:35
@Stefan, see my revised answer for adding multiple directories. – Chris Schmich May 19 '10 at 13:06
Thanks. I think that's the best we can get. – Stefan May 20 '10 at 09:16
8

You should better use `Path.PathSeparator ` – ulrichb Dec 12 '10 at 00:49
Yeah, Path.PathSeparator plus the PATH environment variable worked GREAT! – John Gietzen Jul 30 '13 at 17:50
I added a `Path.PathSeparator` version to my answer. – Chris Schmich Jul 30 '13 at 20:21

score 15 · Answer 2 · edited Jun 11 '13 at 12:08

15

Try calling AddDllDirectory with your additional DLL paths before you call into the imported function for the first time.

If your Windows version is lower than 8 you will need to install this patch, which extends the API with the missing AddDllDirectory function for Windows 7, 2008 R2, 2008 and Vista (there is no patch for XP, though).

edited Jun 11 '13 at 12:08

Pierre Arnaud

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answered May 29 '12 at 13:35

jvrdev

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Sounds like a good idea, but what should the DllImport look like? – Patrick Stalph Oct 06 '16 at 13:50
This is a much better solution because, unlike the accepted answer, this API will work well also when the app is running as Microsoft Store app (UWP or Packaged Win32 app as UWP). The accepted answer will fail for the Microsoft Store app because the OS blocks the list of the current environment to UWP apps. So DllImport will fail to see the path with the dll. To use it in C#, you can copy import this API this way: `[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)] public static extern int AddDllDirectory(string lpPathName);` – gil123 Sep 29 '21 at 19:42

score 5 · Answer 3 · answered Jul 29 '17 at 02:55

5

This might be useful DefaultDllImportSearchPathsAttribute Class
E.g.

[assembly: DefaultDllImportSearchPaths(DllImportSearchPath.UserDirectories)]

Also note you can use AddDllDirectory as well so you aren't screwing up anything already there:

[DllImport("kernel32.dll", SetLastError = true)]
static extern bool AddDllDirectory(string path);

answered Jul 29 '17 at 02:55

Eric

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can you show where to place the [assembly:... attribute ? – IronHide Jun 04 '19 at 05:30
1

I believe it can be anywhere. It applies to the assembly anyway (globally to the .dll). The convention is to put it in the `.\Properties\AssemblyInfo.cs` file where `.` is the project directory. It **might** have to be outside any `namespace` declaration or that is just where it has always been when I encountered it or wrote it myself. – Eric Jun 08 '19 at 01:25

Specify the search path for DllImport in .NET

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