I'm posting because I had this issue and by googling discovered it's a fairly common problem, but no clearcut solutions were described. I puzzled it out from the clues in the error message.

I had the same issue while modifying old C#.net 1.1 framework in Visual Studio .NET 2003. I needed a new textbox control to collect date only input so I created the column in SQL server 2008 as a Date data type then I realized that this framework version didn't recognize the Date (only) data type so I changed it to smalldatetime. Unbeknowst to me at the time, the code initially created the command line to add the parameter using "variant" instead of DateTime.

I would suggest anyone who receives this error should search for the word "variant" in their code, then change it to smalldatetime. For example in the following line of code, change the word "variant" to some other data type:

this.cmdUpdate.Parameters.Add(new System.Data.SqlClient.SqlParameter("@SomeDate", System.Data.SqlDbType.Variant, 4, "SomeDate"));