Visual Studio Forms Designer: The given key 'reader' was not present in the dictionary.

This error indicates a Windows Forms Designer design-time failure, not necessarily a bug in the designer itself. The failure can be caused by code in the control, designer-generated code, missing references, build issues, or a third-party component.

For a case where one minimal user control works and another does not, the most relevant supported causes to check are:

1. The project or control does not build cleanly The designer can fail to load or save when the project has compile-time or type-resolution problems. Clean and rebuild the project first:
   • In Solution Explorer, right-click the project or solution and select Clean.
   • Right-click again and select Rebuild.
2. Temporary build output or dependency state is stale Close Visual Studio, delete the project's bin and obj folders, reopen the solution, and rebuild.
3. A required type cannot be found If the failing control references a type that Visual Studio cannot resolve at design time, the designer can fail. Ensure the assembly containing the type is referenced, and if it is in the current project, ensure the project has been built.
4. The class is not designable in its current form The designer requires a class that can be visually designed. If the class does not inherit from a class that supports designers, or if the base class cannot be resolved, the designer cannot load it.
5. The file structure is preventing the designer from loading the class correctly Visual Studio requires designers to use the first class in a file. If the user control is not the first class in that file, move it so it is first, then reload the designer.
6. Designer-generated code is inconsistent If the problem started after copying, renaming, or editing controls, inspect the .Designer file for mismatched control names in declarations, initialization, property assignments, or event hookups. Correct only obvious mismatches, then rebuild.
7. A property or object used by the designer is invalid The designer can fail if expected objects are null when they must always return an object, or if a variable referenced by designer code is undeclared or never assigned.
8. A third-party component or custom designer is involved Several designer errors are caused by component vendors, including invalid services, toolbox item failures, serializer issues, or unsupported designers.

A practical way to isolate the cause between the two controls is:

• Compare the working and failing control's base class and referenced types.
• Compare their .Designer files for differences in declarations, initialization, and event wiring.
• Build the project and fix any compile errors first.
• Clean/Rebuild, then delete bin and obj if needed.
• If the failing control was copied or renamed, check for inconsistent names in the generated designer code.

The provided documentation does not identify a specific documented cause for the exact message "The given key 'reader' was not present in the dictionary". It supports treating it as a general design-time error and checking code, references, build state, and designer-generated code first.

References:

• Windows Forms Designer error page
• Context Sequence contains no elements - Microsoft Q&A I'm using Visual Studio 2026 writing in VBasic.