You can simply convert between Hijri and Gregorian using the built in CultureInfo class
Imports System.Globalization
Private Sub Convert_From_Hijri_To_Gregorian(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim arCI As CultureInfo = New CultureInfo("ar-SA")
Dim hijri As String = Me.TextBox1.Text '//check if string is valid date first
Dim tempDate As DateTime = DateTime.ParseExact(hijri, "dd/MM/yyyy", arCI.DateTimeFormat, DateTimeStyles.AllowInnerWhite)
Me.TextBox2.Text = tempDate.ToString("dd/MM/yyyy")
End Sub
I am not a C# guy but here is my the conversion of the above code to C#
using System.Globalization;
private void Convert_From_Hijri_To_Gregorian(System.Object sender, System.EventArgs e)
{
CultureInfo arCI = new CultureInfo("ar-SA");
string hijri = TextBox1.Text;
DateTime tempDate = DateTime.ParseExact(hijri, "dd/MM/yyyy", arCI.DateTimeFormat, DateTimeStyles.AllowInnerWhite);
TextBox2.Text = tempDate.ToString("dd/MM/yyyy");
}
I have written this code after reading this article