The assert expression is a debugging feature that you can use to test an expression. Upon failure in Debug mode, an assertion generates a system error dialog box.


assert condition


The assert expression has type bool -> unit.

In the previous syntax, condition represents a Boolean expression that is to be tested. If the expression evaluates to true, execution continues unaffected. If it evaluates to false, a system error dialog box is generated. The error dialog box has a caption that contains the string Assertion Failed. The dialog box contains a stack trace that indicates where the assertion failure occurred.

Assertion checking is enabled only when you compile in Debug mode; that is, if the constant DEBUG is defined. In the project system, by default, the DEBUG constant is defined in the Debug configuration but not in the Release configuration.

The assertion failure error cannot be caught by using F# exception handling.


The assert function resolves to System.Diagnostics.Debug.Assert.


The following code example illustrates the use of the assert expression.

let subtractUnsigned (x : uint32) (y : uint32) =
    assert (x > y)
    let z = x - y
// This code does not generate an assertion failure.
let result1 = subtractUnsigned 2u 1u
// This code generates an assertion failure.
let result2 = subtractUnsigned 1u 2u

See Also

F# Language Reference