One or more selected items contain a data type that is not supported by the designer
Visual Studio 2015
For the latest documentation on Visual Studio 2017, see One or more selected items contain a data type that is not supported by the designer on docs.microsoft.com. One or more of the items dragged from Server Explorer/Database Explorer onto the O/R Designer contains a data type that is not supported by the O/R Designer (for example, CLR User-Defined Types).
To correct this error
Create a view that is based on the desired table and that does not include the unsupported data type.
Drag the view from Server Explorer/Database Explorer onto the designer.
LINQ to SQL Tools in Visual Studio
Walkthrough: Creating LINQ to SQL Classes (O-R Designer)
LINQ to SQL
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