1. Unload project
2. Edit the csproj file.
3. Search for <LastGenOutput>test1.Designer.cs</LastGenOutput>.
4. Correct the value so it shows <LastGenOutput>test.Designer.cs</LastGenOutput>.
5. Delete test.Designer.cs and test1.Designer.cs from disk.
6. Load project again.
Right-click on the dataset and click Run Custom Tool to regenerate the designer file.