add GetRequiredValue to avoid null checks for required options

src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt

@@ -123,6 +123,9 @@
     public System.Collections.Generic.IReadOnlyList<System.String> UnmatchedTokens { get; }
     public System.CommandLine.Completions.CompletionContext GetCompletionContext()
     public System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem> GetCompletions(System.Nullable<System.Int32> position = null)
+    public T GetRequiredValue<T>(Argument<T> argument)
+    public T GetRequiredValue<T>(Option<T> option)
+    public T GetRequiredValue<T>(System.String name)
     public System.CommandLine.Parsing.ArgumentResult GetResult(Argument argument)
     public System.CommandLine.Parsing.CommandResult GetResult(Command command)
     public System.CommandLine.Parsing.OptionResult GetResult(Option option)
@@ -269,6 +272,9 @@ System.CommandLine.Parsing
     public SymbolResult Parent { get; }
     public System.Collections.Generic.IReadOnlyList<Token> Tokens { get; }
     public System.Void AddError(System.String errorMessage)
+    public T GetRequiredValue<T>(Argument<T> argument)
+    public T GetRequiredValue<T>(Option<T> option)
+    public T GetRequiredValue<T>(System.String name)
     public ArgumentResult GetResult(System.CommandLine.Argument argument)
     public CommandResult GetResult(System.CommandLine.Command command)
     public OptionResult GetResult(System.CommandLine.Option option)

src/System.CommandLine.Tests/OptionTests.cs

@@ -215,8 +215,14 @@ public void When_options_use_different_prefixes_they_still_work(string prefix)
             var result = rootCommand.Parse(prefix + "c value-for-c " + prefix + "a value-for-a");
 
             result.GetValue(optionA).Should().Be("value-for-a");
+            result.GetRequiredValue(optionA).Should().Be("value-for-a");
+            result.GetRequiredValue<string>(optionA.Name).Should().Be("value-for-a");
             result.GetResult(optionB).Should().BeNull();
+            result.Invoking(result => result.GetRequiredValue(optionB)).Should().Throw<InvalidOperationException>();
+            result.Invoking(result => result.GetRequiredValue<string>(optionB.Name)).Should().Throw<InvalidOperationException>();
             result.GetValue(optionC).Should().Be("value-for-c");
+            result.GetRequiredValue(optionC).Should().Be("value-for-c");
+            result.GetRequiredValue<string>(optionC.Name).Should().Be("value-for-c");
         }
 
         [Fact]
@@ -243,12 +249,22 @@ public void Option_T_default_value_can_be_set_after_instantiation()
                 DefaultValueFactory = (_) => 123
             };
 
-            new RootCommand { option }
+            var result = new RootCommand { option }
                 .Parse("")
-                .GetResult(option)
+                .GetResult(option);
+
+            result
                 .GetValueOrDefault<int>()
                 .Should()
                 .Be(123);
+
+            result.GetRequiredValue(option)
+                .Should()
+                .Be(123);
+
+            result.GetRequiredValue<int>(option.Name)
+                .Should()
+                .Be(123);
         }
 
         [Fact]
@@ -390,6 +406,8 @@ public void Multiple_identifier_token_instances_without_argument_tokens_can_be_p
             var result = root.Parse("-v -v -v");
 
             result.GetValue(option).Should().BeTrue();
+            result.GetRequiredValue(option).Should().BeTrue();
+            result.GetRequiredValue<bool>(option.Name).Should().BeTrue();
         }
 
         [Fact] 

src/System.CommandLine.Tests/ParserTests.cs

@@ -826,6 +826,33 @@ public void Commands_can_have_default_argument_values()
             GetValue(result, argument)
                   .Should()
                   .Be("default");
+
+            result.GetRequiredValue(argument)
+                  .Should()
+                  .Be("default");
+        }
+
+        [Fact]
+        public void GetRequiredValue_throws_when_argument_without_default_value_was_not_provided()
+        {
+            Argument<int> argument = new("the-arg");
+            Option<bool> option = new("--option");
+
+            Command command = new("command")
+            {
+                argument,
+                option
+            };
+
+            ParseResult result = command.Parse("command --option");
+
+            result.Invoking(result => result.GetRequiredValue(argument))
+                  .Should()
+                  .Throw<InvalidOperationException>();
+
+            result.Invoking(result => result.GetRequiredValue<int>(argument.Name))
+                  .Should()
+                  .Throw<InvalidOperationException>();
         }
 
         [Fact]
@@ -925,6 +952,11 @@ public void Command_default_argument_value_does_not_override_parsed_value()
                   .Name
                   .Should()
                   .Be("the-directory");
+
+            result.GetRequiredValue(argument)
+                  .Name
+                  .Should()
+                  .Be("the-directory");
         }
 
         [Fact]
@@ -1160,6 +1192,10 @@ public void Arguments_can_match_subcommands()
             GetValue(result, argument)
                   .Should()
                   .BeEquivalentSequenceTo("one", "two", "three", "subcommand", "four");
+
+            result.GetRequiredValue(argument)
+                  .Should()
+                  .BeEquivalentSequenceTo("one", "two", "three", "subcommand", "four");
         }
 
         [Theory]

src/System.CommandLine/ParseResult.cs

@@ -133,6 +133,35 @@ CommandLineText is null
         public T? GetValue<T>(string name)
             => RootCommandResult.GetValue<T>(name);
 
+        /// <summary>
+        /// Gets the parsed or default value for the specified required argument or throws.
+        /// </summary>
+        /// <param name="argument">The argument for which to get a value.</param>
+        /// <returns>The parsed value or a configured default.</returns>
+        /// <exception cref="InvalidOperationException">Thrown when required argument was not parsed or has no default value configured.</exception>
+        public T GetRequiredValue<T>(Argument<T> argument)
+            => RootCommandResult.GetRequiredValue(argument);
+
+        /// <summary>
+        /// Gets the parsed or default value for the specified required option or throws.
+        /// </summary>
+        /// <param name="option">The option for which to get a value.</param>
+        /// <returns>The parsed value or a configured default.</returns>
+        /// <exception cref="InvalidOperationException">Thrown when required option was not parsed or has no default value configured.</exception>
+        public T GetRequiredValue<T>(Option<T> option)
+            => RootCommandResult.GetRequiredValue(option);
+
+        /// <summary>
+        /// Gets the parsed or default value for the specified required symbol name, in the context of parsed command (not entire symbol tree).
+        /// </summary>
+        /// <param name="name">The name of the required Symbol for which to get a value.</param>
+        /// <returns>The parsed value or a configured default.</returns>
+        /// <exception cref="InvalidOperationException">Thrown when parsing resulted in parse error(s) or required symbol was not parsed or has no default value configured.</exception>
+        /// <exception cref="ArgumentException">Thrown when there was no symbol defined for given name for the parsed command.</exception>
+        /// <exception cref="InvalidCastException">Thrown when parsed result can not be cast to <typeparamref name="T"/>.</exception>
+        public T GetRequiredValue<T>(string name)
+            => RootCommandResult.GetRequiredValue<T>(name);
+
         /// <inheritdoc />
         public override string ToString() => ParseDiagramAction.Diagram(this).ToString();
 

src/System.CommandLine/Parsing/SymbolResult.cs

@@ -122,6 +122,22 @@ public IEnumerable<ParseError> Errors
             return Argument<T>.CreateDefaultValue();
         }
 
+        /// <inheritdoc cref="ParseResult.GetRequiredValue{T}(Argument{T})"/>
+        public T GetRequiredValue<T>(Argument<T> argument)
+            => GetResult(argument) switch
+            {
+                ArgumentResult argumentResult => argumentResult.GetValueOrDefault<T>(),
+                null => throw new InvalidOperationException($"{argument.Name} is required but was not provided."),
+            };
+
+        /// <inheritdoc cref="ParseResult.GetRequiredValue{T}(Option{T})"/>
+        public T GetRequiredValue<T>(Option<T> option)
+            => GetResult(option) switch
+            {
+                OptionResult optionResult => optionResult.GetValueOrDefault<T>(),
+                null => throw new InvalidOperationException($"{option.Name} is required but was not provided."),
+            };
+
         /// <summary>
         /// Gets the value for a symbol having the specified name anywhere in the parse tree.
         /// </summary>
@@ -147,6 +163,20 @@ public IEnumerable<ParseError> Errors
             return Argument<T>.CreateDefaultValue();
         }
 
+        /// <summary>
+        /// Gets the value for a symbol having the specified name anywhere in the parse tree.
+        /// </summary>
+        /// <param name="name">The name of the symbol for which to find a result.</param>
+        /// <returns>An argument result if the argument was matched by the parser or has a default value; otherwise, <c>null</c>.</returns>
+        public T GetRequiredValue<T>(string name)
+            => GetResult(name) switch
+            {
+                OptionResult optionResult => optionResult.GetValueOrDefault<T>(),
+                ArgumentResult argumentResult => argumentResult.GetValueOrDefault<T>(),
+                SymbolResult _ => throw new InvalidOperationException($"{name} is not an option or argument."),
+                _ => throw new InvalidOperationException($"{name} is required but was not provided."),
+            };
+
         internal virtual bool UseDefaultValueFor(ArgumentResult argumentResult) => false;
     }
 }