The ViewModelBase derives from ModelBase, thus all information here also applies to the ViewModelBase
The easiest way to implement validation is to override the ValidateFields and ValidateBusinessRules methods. Below is an example of an implementation of the ValidateFields method:
protected override void ValidateFields(List<IFieldValidationResult> validationResults)
{
if (string.IsNullOrEmpty(FirstName))
{
validationResults.Add(FieldValidationResult.CreateError(FirstNameProperty, "First name is required"));
}
if (string.IsNullOrEmpty(LastName))
{
validationResults.Add(FieldValidationResult.CreateError(LastNameProperty, "Last name is required"));
}
if (Gender == Gender.Unknown)
{
validationResults.Add(FieldValidationResult.CreateError(GenderProperty, "Gender cannot be unknown"));
}
}