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unchecked (C# Reference)

Updated: February 2009

The unchecked keyword is used to suppress overflow-checking for integral-type arithmetic operations and conversions.

In an unchecked context, if an expression produces a value that is outside the range of the destination type, the overflow is not flagged. For example, because the calculation in the following example is performed in an unchecked block or expression, the fact that the result is too large for an integer is ignored, and int1 is assigned the value -2,147,483,639:

    int1 = 2147483647 + 10;
int1 = unchecked(ConstantMax + 10);

If the unchecked environment is removed, a compilation error occurs. The overflow can be detected at compile time because all the terms of the expression are constants.

Expressions that contain non-constant terms are unchecked by default at compile time and run time. See checked (C# Reference) for information about enabling a checked environment.

Because checking for overflow takes time, the use of unchecked code in situations where there is no danger of overflow might improve performance. However, if overflow is a possibility, a checked environment should be used.

This sample shows how to use the unchecked keyword.

class UncheckedDemo
    static void Main(string[] args)
        // int.MaxValue is 2,147,483,647. 
        const int ConstantMax = int.MaxValue;
        int int1;
        int int2;
        int variableMax = 2147483647;

        // The following statements are checked by default at compile time. They do not 
        // compile. 
        //int1 = 2147483647 + 10; 
        //int1 = ConstantMax + 10; 

        // To enable the assignments to int1 to compile and run, place them inside  
        // an unchecked block or expression. The following statements compile and 
        // run. 
            int1 = 2147483647 + 10;
        int1 = unchecked(ConstantMax + 10);

        // The sum of 2,147,483,647 and 10 is displayed as -2,147,483,639.

        // The following statement is unchecked by default at compile time and run  
        // time because the expression contains the variable variableMax. It causes   
        // overflow but the overflow is not detected. The statement compiles and runs.
        int2 = variableMax + 10;

        // Again, the sum of 2,147,483,647 and 10 is displayed as -2,147,483,639.

        // To catch the overflow in the assignment to int2 at run time, put the 
        // declaration in a checked block or expression. The following 
        // statements compile but raise an overflow exception at run time. 
            //int2 = variableMax + 10;
        //int2 = checked(variableMax + 10); 

        // Unchecked sections frequently are used to break out of a checked  
        // environment in order to improve performance in a portion of code  
        // that is not expected to raise overflow exceptions. 
            // Code that might cause overflow should be executed in a checked 
            // environment. 
                // This section is appropriate for code that you are confident  
                // will not result in overflow, and for which performance is  
                // a priority.
            // Additional checked code here. 

For more information, see the following sections in the C# Language Specification:

  • Block statements, checked, and unchecked statements

  • 7.5.12 The checked and unchecked operators

  • 8.11 The checked and unchecked statements




February 2009

Clarified default behavior and performance implications.

Content bug fix.