Question

What does ? mean:

public bool? Verbose { get; set; }

When applied to string?, there is an error:

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’

Answer 1

? makes your non-nullable (value) types nullable. It doesn’t work for string, as it is reference type and therefore nullable by default.

From MSDN, about value types:

Unlike reference types, a value type
cannot contain the null value.
However, the nullable types feature
does allow for values types to be
assigned to null.

? is basically a shorthand for Nullable<T> structure.

If you want to know more, MSDN has a great article regarding this topic.