A question mark in the definition of a C# variable indicates that the variable is allowed to be empty.

Nullable types can represent null values of the underlying type, or they can represent. null？ Value. ? Types that can be null can be declared in one of two ways:

System. Can be empty & ltT>? Variable-or-

t？ variableT？ Is the base type of nullable type. ? t？ Can it be included? struct？ Any value type, including: but it cannot be a reference type.

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You can view MSDN null: nullable types (C# programming guide) and using nullable types (C# programming guide).