The resource name '<file>' is not a valid identifier
This warning occurs if the resource name is not a valid identifier (for example, it contains a space), and strongly typed resource generation is turned on for the file. This is a warning instead of an error because the strongly typed resource name will be changed to a valid name (for example, "a b" will be changed to "a_b"), but the resource name will remain "a b."
To correct this error
You should first change the name of the resource to a valid identifier, if possible. If that is not a valid option, disable strongly typed resource code generation for this .resx file. To do this, select the .resx file in Solution Explorer and clear the Custom Tool property.