In an unchecked context, if an expression produces a value that is outside the range of the destination type, the result is truncated. For example:

        unchecked
{
    int val = 2147483647 * 2;
}

Because the calculation above is performed in an unchecked block, the fact that the result is too large for an integer is ignored, and val is assigned the value -2. By default, overflow detection is enabled, which has the same effect as using checked.

In the example above, had unchecked been omitted, a compilation error would occur because the expression uses constants and the result is known at compile time. The unchecked keyword also suppresses overflow detection for non-constant expressions, which otherwise result in OverflowException at runtime.

The unchecked keyword can also be used as an operator, like this:

public int UncheckedAdd(int a, int b)
{
    return unchecked(a + b);
}

This sample shows how to use the unchecked statement, using unchecked with constant expressions.

// statements_unchecked.cs
using System;

class TestClass 
{
    const int x = 2147483647;   // Max int 
    const int y = 2;

    static void Main() 
    {
        int z;
        unchecked 
        {
            z = x * y;
        }
        Console.WriteLine("Unchecked output value: {0}", z);
    }
}

Output

Unchecked output value: -2

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

• 5.3.3.2 Block statements, checked, and unchecked statements

• 7.5.12 The checked and unchecked operators

• 8.11 The checked and unchecked statements

Reference

Concepts

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