The return value from a stored procedure gets the first character only in ASP.NET

Make sure you really use it like this:

@Result NVARCHAR(8) OUTPUT
SqlParameter resultValue = new SqlParameter("@Result", SqlDbType.NVarChar, 8);

The default length for (N)VARCHAR columns is 1.

According to MSDN:

For output parameters with a variable length type (nvarchar, for example), the size of the parameter defines the size of the buffer holding the output parameter. The output parameter can be truncated to a size specified with Size.

So it's important to specify the size for out parameters.