CustomValidation Annotation Event Raise Issue MVVM Silverlight 5 - silverlight

Question

I've Created a Model which have few custom validation. These custom validation I've annotated at property by below code
[CustomValidation(typeof(ItemmasterModel), "ValueTextMaxLenghtValidate")]
public decimal Valuetextmaxlength
{
get
{
return _Valuetextmaxlength;
}
set
{
ValidateProperty("Valuetextmaxlength",value);
_Valuetextmaxlength = value;
RaisePropertyChanged(() => Valuetextmaxlength);
}
}
public static ValidationResult ValueTextMaxLenghtValidate(object obj, ValidationContext context)
{
var itmmstr = (ItemmasterModel)context.ObjectInstance;
if (itmmstr.SelectedValuetypeDd != null)
{
string vtype = itmmstr.SelectedValuetypeDd.Key.ToString();
if (vtype.Equals("C"))
{
if (itmmstr.SelectedItemValueCodeTypesDd != null)
{
string vcode = itmmstr.SelectedItemValueCodeTypesDd.Key.ToString();
if (vcode.Equals("T"))
{
if (itmmstr.Valuetextmaxlength == null || itmmstr.Valuetextmaxlength == 0)
{
return new ValidationResult("Value Max Length is not Entered",
new List<string> { "Valuetextmaxlength" });
}
}
}
}
else if (vtype.Equals("T"))
{
if (itmmstr.Valuetextmaxlength == null || itmmstr.Valuetextmaxlength == 0)
{
return new ValidationResult("Value Max Length is not Entered",
new List<string> { "Valuetextmaxlength" });
}
}
}
return ValidationResult.Success;
}
Now this validation code depend on other property. scenerio When User select a value from dropdown it makes 1 checkbox selected automatically and User should enter the value in texbox also.
Issue:
Validation is working. checkbox is selected at first time then also it comes with error popup.
untill user doesn't make changes into this checkbox or texbox it is with error only. 1 time it says error even value has been entered .Next time it goes even user have not entered anything but during final full object validation is again comes with error.
Why this even ambiguity is happening. How to solve this.
Need more code let me know. I'll Post. Code is in Silverlight 5, MVVM Light

Answer 1

During my Custom validation I was using ValidateProperty("Valuetextmaxlength",value); before setting the value of property so It was giving me issue but the moment I set the value first and then Use my ValidateProperty("Valuetextmaxlength",value); everything worked smooth. Still I don't know the reason why but It worked for me.