Transforming Values


The methods documented below are no longer recommended or supported and will be removed in FluentValidation 12. We instead recommend using computed properties on your model if you need to perform a transformation. For details please see this GitHub issue

As of FluentValidation 9.5, you can apply a transformation to a property value prior to validation being performed against it. For example, if you have property of type string that actually contains numeric input, you could apply a transformation to convert the string value to a number.

Transform(from: x => x.SomeStringProperty, to: value => int.TryParse(value, out int val) ? (int?) val : null)

This rule transforms the value from a string to a nullable int (returning null if the value couldn’t be converted). A greater-than check is then performed on the resulting value.

Syntactically this is not particularly nice to read, so the logic for the transformation can optionally be moved into a separate method:

Transform(x => x.SomeStringProperty, StringToNullableInt)

int? StringToNullableInt(string value)
  => int.TryParse(value, out int val) ? (int?) val : null;

This syntax is available in FluentValidation 9.5 and newer.

There is also a TransformForEach method available, which performs the transformation against each item in a collection.