Fixing FileNotFoundException for External DLLs in .NET F# Projects

Learn how to resolve the common FileNotFoundException when running .NET F# applications with external DLL references by properly managing dependencies and output directories.
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This video is based on the question https://stackoverflow.com/q/79421561/ asked by the user 'Пуйло-Хуйло сдохни гнида' ( https://stackoverflow.com/u/5688082/ ) and on the answer https://stackoverflow.com/a/79423972/ provided by the user 'Pac0' ( https://stackoverflow.com/u/479251/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Problem: Runtime Errors Despite Successful Compilation

When adding an external DLL (e.g., internalLib.dll) to your F# Console Application project, you might notice:

Intellisense and auto-completion work perfectly.

The open internalLib statement compiles without errors.

However, at runtime, you encounter an error like:

[[See Video to Reveal this Text or Code Snippet]]

This occurs even though the DLL path in your project file is correct.



Root Cause

Your referenced DLL (internalLib.dll) likely depends on other assemblies or native libraries that are missing from your application's output directory (e.g., /bin/Debug or /bin/Release). The runtime loader cannot resolve these dependencies, causing the failure.

Simply adding the main DLL reference doesn't copy its dependencies to the output folder automatically.



Solutions

1. Ensure All Dependencies Are Present at Runtime

You must have all dependent DLLs located alongside your main executable or in the probing paths.

2. Manual Copy (Quick Fix)

Copy the required DLLs manually to your project's output folder (/bin/Debug or /bin/Release).

This works but is error-prone and not maintainable.

3. Automated Copy in Visual Studio (Recommended)

Steps:

Create a folder in your project or solution (e.g., libs/) and place all external DLLs—including dependencies—there.

In Visual Studio, include these DLL files in your project (Add Existing Item).

Select each DLL in Solution Explorer, then set the Copy to Output Directory property to either:

Copy if newer

or Copy always

This ensures that all necessary assemblies will be copied automatically to the output directory whenever you build, keeping your runtime environment consistent.

4. Reference Dependency Projects or NuGet Packages

If dependencies are available as projects or NuGet packages:

Add them as project references or package dependencies.

This automatically manages buildup and copying.

Saves you from manual file management.



Summary

Runtime loading errors are usually due to missing dependent DLLs, not just the main referenced DLL.

Confirm all dependencies are in the runtime output folder.

Use Visual Studio's Copy to Output Directory property for robust, automated handling.

Prefer NuGet or project references where possible to manage dependencies better.

This approach will help you avoid FileNotFoundException issues and achieve smoother development and deployment.