Nullable generic field in generic class

问题

I'm trying to do something like this:

public class MySuperCoolClass<T>
{
    public T? myMaybeNullField {get; set;}
}

Is this possible?

This gives me the error:

error CS0453: The type T' must be a non-nullable value type in order to use it as type parameterT' in the generic type or method System.Nullable'.

Thanks

回答1:

Add where T : struct generic constraint to get rid of the error since Nullable<T> accepts only struct.

public class MySuperCoolClass<T> where T : struct
{
    public T? myMaybeNullField { get; set; }
}

Nullable<T> is defined as below

public struct Nullable<T> where T : struct

So you're also forced to do so, just to prevent you from doing MySuperCoolClass<object> which makes object? which is not valid.

回答2:

Do you see your job ?

public static Nullable<T> ToNullable<T>(this string s) where T: struct
{
    Nullable<T> result = new Nullable<T>();
    try
    {
        if (!string.IsNullOrEmpty(s) && s.Trim().Length > 0)
        {
            TypeConverter conv = TypeDescriptor.GetConverter(typeof(T));
            result = (T)conv.ConvertFrom(s);
        }
    }
    catch { } 
    return result;
}

来源：https://stackoverflow.com/questions/20353742/nullable-generic-field-in-generic-class