If you have multiple fields to check, simply make a small helper method that returns a string (or empty) based on a condition (boolean):
messageErreur += Check(!myField.IsNullOrNa(), myField.IsDecimalWithMessage(nameof(myField));
messageErreur += Check(myField.HasError(), myField.HasError(nameof(myField));
private string Check(bool condition, string message) => condition ? message : string.Empty;
0
solved Convert if condition into ternary with C#