Working calculator

Server/Calculator.cs

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+using System.Globalization;
+
+namespace GrpcCalculator.Server;
+
+public enum CalculatorState
+{
+    AfterEquals,
+    AfterOperator,
+    AfterDigit,
+}
+
+
+public enum CalculatorOperator
+{
+    Addition,
+    Subtraction,
+    Multiplication,
+    Division,
+}
+
+public static class CalculatorOperatorMethods
+{
+    public static string Evaluate(this CalculatorOperator op, string leftOperand, string rightOperand)
+    {
+        var leftOperandValue = double.Parse(leftOperand);
+        var rightOperandValue = double.Parse(rightOperand);
+
+        return op switch
+        {
+            CalculatorOperator.Addition => (leftOperandValue + rightOperandValue).ToString(CultureInfo.InvariantCulture),
+            CalculatorOperator.Subtraction => (leftOperandValue - rightOperandValue).ToString(CultureInfo.InvariantCulture),
+            CalculatorOperator.Multiplication => (leftOperandValue * rightOperandValue).ToString(CultureInfo.InvariantCulture),
+            CalculatorOperator.Division => (leftOperandValue / rightOperandValue).ToString(CultureInfo.InvariantCulture),
+        };
+    }
+}
+
+public record Calculator(CalculatorState State, string LeftOperand, CalculatorOperator? Operator, string RightOperand)
+{
+    private string Evaluate() => Operator.HasValue switch {
+        true => Operator.Value.Evaluate(LeftOperand, RightOperand),
+        false => RightOperand,
+    };
+
+    public Calculator() : this(CalculatorState.AfterEquals, "0", null, "0") { }
+
+    public Calculator OperatorPressed(CalculatorOperator op) => this with
+    {
+        LeftOperand = State switch {
+            CalculatorState.AfterOperator or CalculatorState.AfterEquals => RightOperand,
+            CalculatorState.AfterDigit => Evaluate(),
+        },
+        Operator = op,
+        State = CalculatorState.AfterOperator,
+    };
+
+    public Calculator EqualsPressed() => this with {
+        LeftOperand = Evaluate(),
+        State = CalculatorState.AfterEquals,
+    };
+
+    public Calculator DigitPressed(string digit) => this with {
+        LeftOperand = State switch {
+            CalculatorState.AfterEquals or CalculatorState.AfterOperator => RightOperand,
+            CalculatorState.AfterDigit => LeftOperand,
+        },
+        RightOperand = State switch {
+            CalculatorState.AfterOperator or CalculatorState.AfterEquals => digit,
+            CalculatorState.AfterDigit => RightOperand + digit,
+        },        
+        State = CalculatorState.AfterDigit,
+    };
+
+    public string Display =>
+        State switch
+        {
+            CalculatorState.AfterDigit => RightOperand,
+            CalculatorState.AfterOperator or CalculatorState.AfterEquals => LeftOperand,
+        };
+}

Server/Program.cs

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+// See https://aka.ms/new-console-template for more information
+Console.WriteLine("Hello, World!");

Server/Server.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <RootNamespace>GrpcCalculator.Server</RootNamespace>
+    <AssemblyName>GrpcCalculator.Server</AssemblyName>
+  </PropertyGroup>
+
+</Project>