System.DllNotFoundException – Unable to load DLL ‘xyz’

The System.DllNotFoundException with the message “Unable to load DLL ‘xyz'” occurs in C# when the runtime is unable to locate or load a native DLL (dynamic-link library) that your application depends on. This typically happens due to issues such as the DLL being missing, having an incorrect path, or being incompatible with the current platform.

Common Causes and Solutions

1. Missing DLL:
   If the required native DLL is not present in the application’s directory or system path, this exception will occur.

   [DllImport("MyNativeLibrary.dll")]
   public static extern void MyNativeMethod();

   MyNativeMethod(); // DllNotFoundException – DLL not found

Fix: Ensure the DLL is present in the application’s directory or a directory in the system’s PATH.

   // Copy MyNativeLibrary.dll to the output directory or a directory in the PATH

2. Incorrect DLL Name or Path:
   If the DLL name or path specified in the DllImport attribute is incorrect, this exception will occur.

   [DllImport("WrongLibraryName.dll")]
   public static extern void MyNativeMethod(); // DllNotFoundException – Incorrect DLL name

Fix: Verify the DLL name and path.

   [DllImport("MyNativeLibrary.dll")]
   public static extern void MyNativeMethod(); // Correct DLL name

3. Platform-Specific DLLs:
   If the DLL is platform-specific (e.g., x86 vs x64) and the wrong version is used, this exception can occur. Fix: Ensure the correct platform-specific DLL is used.

   // Use the correct DLL for the target platform (e.g., MyNativeLibrary_x86.dll or MyNativeLibrary_x64.dll)
   [DllImport("MyNativeLibrary_x64.dll")]
   public static extern void MyNativeMethod();

4. DLL Dependencies:
   If the native DLL depends on other DLLs that are missing, this exception can occur. Fix: Ensure all dependencies of the native DLL are available.

   // Use tools like Dependency Walker to identify and resolve missing dependencies

5. Environment Variables:
   If the DLL is located in a directory that is not in the system’s PATH, this exception can occur. Fix: Add the directory containing the DLL to the system’s PATH environment variable.

   // Add the directory to the PATH environment variable
   Environment.SetEnvironmentVariable("PATH", Environment.GetEnvironmentVariable("PATH") + ";C:\Path\To\DLL");

6. 32-bit vs 64-bit Mismatch:
   If your application is running as a 64-bit process but the DLL is 32-bit (or vice versa), this exception can occur. Fix: Ensure the application and DLL match in terms of 32-bit or 64-bit.

   // Set the platform target in your project to match the DLL (e.g., x86 or x64)
   <PlatformTarget>x64</PlatformTarget>

7. DLL Loading Order:
   If the DLL is loaded in a specific order and dependencies are not resolved correctly, this exception can occur. Fix: Use SetDllDirectory or AddDllDirectory to control the DLL search path.

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

   SetDllDirectory("C:\Path\To\DLL");

8. DLL Redistribution:
   If the DLL requires redistributable packages (e.g., Visual C++ Redistributable), this exception can occur if the packages are not installed. Fix: Install the required redistributable packages.

   // Install the Visual C++ Redistributable for the required version