Добавьте файлы проекта.

Services/Caculyator.cs

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+using System.Globalization;
+
+namespace calculator.Services
+{
+    internal class Caculyator : ICaculator
+    {
+        public string Calculate(string n, string m, string operation)
+        {
+            if (double.TryParse(n, out var numberone) == false)
+                return string.Empty;
+            if (double.TryParse(m, out var numbertwo) == false)
+                return string.Empty;
+
+            return ApplyOperator(operation, numberone, numbertwo).ToString("0.#####");
+        }
+
+        public string SingleOperation(string operand, string operation)
+        {
+            if (double.TryParse(operand, out var numberone) == false)
+                return string.Empty;
+            return ApplyOperator(operation, numberone).ToString("0.#####");
+        }
+
+        private static double ApplyOperator(string token, double operand1, double operand2)
+        {
+            switch (token)
+            {
+                case "+":
+                    return operand1 + operand2;
+                case "-":
+                    return operand1 - operand2;
+                case "*":
+                    return operand1 * operand2;
+                case "/":
+                    return operand1 / operand2;
+                case "^":
+                    return Math.Pow(operand1, operand2);
+                default:
+                    throw new ArgumentException($"Invalid operator: {token}");
+            }
+        }
+
+        private static double ApplyOperator(string token, double operand1)
+        {
+            switch (token)
+            {
+                case "1/":
+                    return 1 / operand1;
+                case "√":
+                    return Math.Sqrt(operand1);
+                case "%":
+                    return operand1 / 100;
+                case "-1":
+                    return operand1 * -1;
+                default:
+                    throw new ArgumentException($"Invalid operator: {token}");
+            }
+        }
+    }
+}

Services/ICaculator.cs

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+namespace calculator.Services;
+
+public interface ICaculator
+{
+    string Calculate(string op1, string op2, string operation);
+    string SingleOperation(string operand, string operation);
+}

Services/IInputService.cs

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+namespace calculator.Services
+{
+    public interface IInputService
+    {
+        public string TryInput(string input);
+    }
+}

Services/InputService.cs

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+namespace calculator.Services
+{
+    internal class InputService : IInputService
+    {
+        public string TryInput(string input)
+        {
+            //если есть знак "бесконечность" - не даёт писать цифры после него
+            if (input.Contains('∞'))
+            {
+                input = input[..^1];
+            }
+
+            //ситуация: слева ноль, а после него НЕ запятая, тогда ноль можно удалить
+            if (input[0] == '0' && (input.IndexOf(".", StringComparison.Ordinal) != 1))
+            {
+                input = input.Remove(0, 1);
+            }
+
+            return input;
+        }
+
+    }
+}