Is it possible to tell Visual Studio not to treat a source file as a "component"?

Can't you use the DesignerCategory attribute to decorate your class ?

When decorating your class with this attribute like displayed below, the file should open in 'code view' when you double click it:

[System.ComponentModel.DesignerCategory("Code")]
public class MyComponent
{
}

As OregonGhost already mentioned in his comment this also happens if you make a partial class file from a form.

The problem is even more worse. If you take a look into your project file you can find entries for every file include like this:

<Compile Include="FormMain.cs">
  <SubType>Form</SubType>
</Compile>

So the solution should be to delete the line <SubType>...</SubType> cause it seems to be the root of all the problems. But if you delete this line, save the file, open it in Visual Studio, save it again and take a look again into, the line will reappear!

There seems to be only one hard-coded exception within Visual Studio and this is *.Designer.cs. So there is no solution to accomplish this problem.