What Is Question Mark After Data Type C#?


Scott Campbell

What Is Question Mark After Data Type C#?

In C#, the question mark (?) after a data type is used to declare a nullable value type.

Nullable value types were introduced in C# 2.0 and provide a way to assign null values to value types, which are typically non-nullable.

Nullable Value Types

Value types in C# are typically non-nullable, meaning they cannot be assigned null values. For example, if you declare an integer variable without the question mark (?), like this:

    int age;

You cannot assign null to this variable, as it will result in a compilation error. However, by adding the question mark after the data type:

    int? age;

You can now assign null to this variable by explicitly setting it:

    age = null;

Working with Nullable Value Types

When you declare a nullable value type, you introduce additional behavior and considerations when working with the variable.

  • HasValue Property: The nullable value type provides a HasValue property that returns true if the variable has a value and false if it is null.
  • Value Property: To access the underlying value of a nullable value type, you use the Value property. However, if the variable is null, accessing this property will throw an exception. It is recommended to check if the variable has a value using the HasValue property before accessing Value.
  • Null Coalescing Operator: The null coalescing operator (??)

    can be used to provide a default value when a nullable value type is null. For example, int age = nullableAge ?? 0; assigns 0 to age if nullableAge is null.

  • Nullable Reference Types: Starting from C# 8.0, reference types can also be made nullable by enabling the nullable reference types feature. This allows reference types to have a null value by default and adds more safety in avoiding null reference exceptions.

Benefits of Nullable Value Types

The introduction of nullable value types in C# provides several benefits:

  • Better Representation of Missing Values: Nullable value types allow for a more accurate representation of missing or unknown values in your code.
  • Avoidance of Boxing and Unboxing: In cases where you would typically use a value type but also need to handle null values, using a nullable value type can help avoid unnecessary boxing and unboxing operations.
  • Easier Integration with Databases and APIs: Many databases and APIs use null values to indicate missing or unknown data. Nullable value types make it easier to work with these external systems.

Caveats of Nullable Value Types

While nullable value types are powerful, there are some caveats to consider:

  • Potential Null Reference Exceptions: If you forget to check if a nullable value type is null before accessing its underlying value, you may encounter a NullReferenceException at runtime.
  • Additional Memory Overhead: Nullable value types require additional memory compared to their non-nullable counterparts. This additional overhead can be a concern in scenarios where memory usage is critical.


The question mark (?) after a data type in C# allows you to declare nullable value types, which can store null values. This feature provides flexibility when working with value types and improves the representation of missing or unknown values in your code.

However, it is important to handle nullable value types correctly to avoid null reference exceptions and consider the additional memory overhead. Understanding nullable value types is essential for writing robust and flexible C# code.

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