Why Does My Code Throw a TypeLoadException Even Though the Method Exists?

Understanding TypeLoadExceptions and Missing Implementations

Developers often encounter the frustrating TypeLoadException: "Method 'SetShort' in type 'DummyItem' ... does not have an implementation," even when the method clearly exists. This article explores the root cause and solution.

The Puzzle:

The error message is misleading. The SetShort method is present in the DummyItem class. The confusion deepens when the calling code doesn't even use SetShort.

The Solution: Clean Build and Assembly Versioning

Quick Fix: Delete all bin and obj folders in your solution and perform a full rebuild. This synchronizes assembly versions.

Detailed Explanation:

The problem stems from version mismatches between assemblies. This commonly happens when an interface (e.g., InterfaceDef) is updated to include a new method (like SetShort), but the implementing class (DummyItem in Implementation assembly) isn't recompiled with the updated interface. Even though SetShort exists in DummyItem, the runtime doesn't recognize the updated interface definition, leading to the exception. The crucial link between the interface and its implementation is broken.

Illustrative Example:

Imagine three projects:

1. InterfaceDef: Defines the interface containing SetShort.
2. Implementation: Contains DummyItem, which implements the interface.
3. ClientCode: Uses classes from Implementation.

If SetShort is added to InterfaceDef but Implementation isn't rebuilt with the updated InterfaceDef reference, running ClientCode will trigger the TypeLoadException when DummyItem is instantiated.

Conclusion:

This TypeLoadException highlights the importance of consistent assembly versions. A clean rebuild resolves version conflicts, ensuring that the runtime correctly links interfaces and their implementations. Thoroughly cleaning your build directories is crucial for preventing these subtle yet disruptive errors.

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