类型'string'必须是不可为空的值类型，以便在泛型类型或方法'System.Nullable <T>'中将其用作参数'T'

Question

public virtual int Fill(DataSetservices.Jobs_detailsDataTable dataTable, 
    global::System.Nullable<global::System.String> fromdate,     
    global::System.Nullable<global::System.DateTime> todate)

I wrote above code in dataset.xsd in C#, but it is throwing an error:

Error 1
The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

Suggest me how to use string because i want to use string and nothing else

Answer 1

string is already nullable, because it's a reference type. You don't need to wrap it in Nullable in order to have a null value. Not only is it not needed, but as per the error message you're getting, it's not even possible . Only non-nullable value types can be used as the generic argument for Nullable .

Answer 2

String Class is a class, it not a struct like System.Int32 or other primitive types. It can hold null value. Nullable<T> works with value types.

From the name it appears that you want to store DateTime object. Its always better to have DateTime in its own type ie. DateTime , for Nullable you can use Nullable<DateTime> or DateTime?

Answer 3

If you look at the docs on MSDN about Nullable<T> you will notice that T is constrained with struct. Constraints on Type parameters reveals that such a constraint restricts the generic parameter to being a value type except Nullable<T> (Do note that Nullable<T> is a struct!).

As MSDN docs say, string is a reference type meaning that the generic parameter constraint made for Nullable<T> will invalidate types such as Nullable<string> or with any reference type as generic parameter.