Error "CS0104: 'DataType' is an ambiguous reference between 'System.ComponentModel.DataAnnotations.DataType' and 'CarlosAg.ExcelXmlWriter.DataType"

asp.net, c#

Solution

As described in the documentation on Compiler Error CS0104 you've got a symbol clash - there are two classes in scope of your source file which are both called `DataType` - one is in the namespace `System.ComponentModel.DataAnnotations` and the other is in the namespace `CarlosAg.ExcelXmlWriter.DataType'`.

You need to do one of the following to resolve this:

1. Explicitly provide the full namespace prefix on each usage, i.e. `System.ComponentModel.DataAnnotations.DataType` and `CarlosAg.ExcelXmlWriter.DataType`:

var cdt = new CarlosAg.ExcelXmlWriter.DataType();
var sdt = new System.ComponentModel.DataAnnotations.DataType();

OR 2. Or use a using directive to alias the namespaces / types, e.g.

using SystemDT = System.ComponentModel.DataAnnotations;
using Carlos = CarlosAg.ExcelXmlWriter;

And then then identify the types with the namespace aliases, e.g..

var dt = new Carlos.DataType();

OR 3. You can also alias at the class level:

using SystemDataType = System.ComponentModel.DataAnnotations.DataType;
using CarlosDataType = CarlosAg.ExcelXmlWriter.DataType;
...
var myObj = new CarlosDataType();

OR 4. If you don't need to symbols from both namespaces, then delete the unused namespace from the `using` clause.

My preference would be for Option 2 - it makes it clearer to the reader that there is a namespace clash, without being too verbose (like Option 1 is)

Edit

Re: "I tried by giving full prefix but still I am getting error "CS0138: A using namespace directive can only be applied to namespaces; 'CarlosAg.ExcelXmlWriter.DataType' is a type not a namespace"

(All relating to point #2, above). The error message refers to a situation like this, which isn't permitted in .Net (but is permitted in Java imports)

// i.e. This won't work, can't import at a class level unless it is aliased
using System.ComponentModel.DataAnnotations.DataType; 

As per my answer, I would recommend that you alias the namespace, and then use the alias prefix to disambiguate between the 2 `DataType`s

Problem

Original source