An expression in C# syntax (see below for details).
The error message to raise if Expression evaluates to false.
Refer to If C# for examples of Expression. This Node also support Custom Properties which can be referenced in the Expression property.
Expression should be a C#-styled expression. This syntax requires explicit handling of casting and potential null values. If you do not need precise control over casting and null handling, consider using Validate for a VB.Net-styled expression.