support for .NET framework 3.5

Still have bugs

Net35/HandyControlDemo_Net35/Tools/Converter/StringRepeatConverter.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Globalization;
+using System.Text;
+using System.Windows.Data;
+
+namespace HandyControlDemo.Tools.Converter
+{
+    public class StringRepeatConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (value is string strValue)
+            {
+                var builder = new StringBuilder();
+                int num;
+                if (parameter is string numStr)
+                {
+                    if (!int.TryParse(numStr, out num))
+                    {
+                        return strValue;
+                    }
+                }
+                else if(parameter is int intValue)
+                {
+                    num = intValue;
+                }
+                else
+                {
+                    return strValue;
+                }
+                for (var i = 0; i < num; i++)
+                {
+                    builder.Append(strValue);
+                }
+                return builder.ToString();
+            }
+            return string.Empty;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}