When the member type is numeric as in your case, RadDataFilter will instantiate a RadMaskedText as its filter editor.
Unfortunately, the RadMaskedTextBox works only with the System.Double value type for handling numeric and currency masks. So what it does is convert the Int64 to a Double.
Precision will always be lost, as it uses double.ToString(this.Mask) to form the masked text. Parsing the masked text (on the way back) is done by double.Parse(…), which will also impact the precision.
If you were using Int32, this should not be a problem.
Currently, the only way to overcome this limitation of RadMaskedTextBox, will be to create a simple filter editor of your own. It can be a plain old TextBox that can handle long (Int64) values.
Here is an online demo that demonstrates how you can supply any kind of user control for the filter editor of RadDataFilter. You will supply a normal TextBox and you will set an IValueConverter on its Text-to-Value Binding that will convert the string that the user enters into an Int64. This should do the job.
Please, excuse us for the inconvenience.
the Telerik team
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