Use array instead of substring

src/CalculatorUITestFramework/HistoryItem.cs

@@ -17,12 +17,12 @@ namespace CalculatorUITestFramework
         public string GetValue()
         {
             var equalSignIndex = Item.Text.IndexOf("=");
-            return Item.Text.Substring(equalSignIndex + 1).Trim();
+            return Item.Text[(equalSignIndex + 1)..].Trim();
         }
         public string GetExpression()
         {
             var equalSignIndex = Item.Text.IndexOf("=");
-            return Item.Text.Substring(0, equalSignIndex + 1).Trim();
+            return Item.Text[..(equalSignIndex + 1)].Trim();
         }
     }
 }

src/CalculatorUITestFramework/NumberPad.cs

@@ -34,7 +34,7 @@ namespace CalculatorUITestFramework
             string numberStr = number.ToString(CultureInfo.InvariantCulture);
             if (numberStr.StartsWith("-"))
             {
-                numberStr = numberStr.Substring(1) + "-";
+                numberStr = numberStr[1..] + "-";
             }
             foreach (char digit in numberStr)
             {
@@ -77,7 +77,7 @@ namespace CalculatorUITestFramework
                         this.NegateButton.Click();
                         break;
                     default:
-                        throw (new ArgumentException(string.Format("{0} is not valid", digit)));
+                        throw (new ArgumentException($"{digit} is not valid"));
                 }
             }
         }