Symptoms
You are seeing the following error in the console window:
error CS0234: The type or namespace `_____' does not exist in the namespace `_____'. Are you missing an assembly reference?
Cause
This error is caused when you try to access a type or namespace that does not exist within a valid namespace.
Error this example presents:
error CS0234: The type or namespace `DateTim' does not exist in the namespace `System'. Are you missing an assembly reference?
Resolution
Ensure that the type or namespace you are trying to use derives from the namespace that proceeds it.
More Information
https://msdn.microsoft.com/en-us/library/0e92xd7b.aspx