Difference between Parameters.Add(string, object) and Parameters.AddWithValue

There is no difference in terms of functionality. In fact, both do this:

return this.Add(new SqlParameter(parameterName, value));

The reason they deprecated the old one in favor of AddWithValue is to add additional clarity, as well as because the second parameter is object, which makes it not immediately obvious to some people which overload of Add was being called, and they resulted in wildly different behavior.

Take a look at this example:

 SqlCommand command = new SqlCommand();
 command.Parameters.Add("@name", 0);

At first glance, it looks like it is calling the Add(string name, object value) overload, but it isn't. It's calling the Add(string name, SqlDbType type) overload! This is because 0 is implicitly convertible to enum types. So these two lines:

 command.Parameters.Add("@name", 0);

and

 command.Parameters.Add("@name", 1);

Actually result in two different methods being called. 1 is not convertible to an enum implicitly, so it chooses the object overload. With 0, it chooses the enum overload.

The difference is the implicit conversion when using AddWithValue. If you know that your executing SQL query (stored procedure) is accepting a value of type int, nvarchar, etc, there's no reason in re-declaring it in your code.

For complex type scenarios (example would be DateTime, float), I'll probably use Add since it's more explicit but AddWithValue for more straight-forward type scenarios (Int to Int).

Without explicitly providing the type as in command.Parameters.Add("@ID", SqlDbType.Int);, it will try to implicitly convert the input to what it is expecting.

The downside of this, is that the implicit conversion may not be the most optimal of conversions and may cause a performance hit.

There is a discussion about this very topic here: http://forums.asp.net/t/1200255.aspx/1