implement nullable prompt

src/Spectre.Console/Extensions/AnsiConsoleExtensions.Prompt.cs

@@ -22,6 +22,23 @@ public static T Prompt<T>(this IAnsiConsole console, IPrompt<T> prompt)
         return prompt.Show(console);
     }
 
+    /// <summary>
+    /// Displays a prompt to the user.
+    /// </summary>
+    /// <typeparam name="T">The prompt result type.</typeparam>
+    /// <param name="console">The console.</param>
+    /// <param name="prompt">The prompt to display.</param>
+    /// <returns>The prompt input result.</returns>
+    public static T? PromptNullable<T>(this IAnsiConsole console, INullablePrompt<T> prompt)
+    {
+        if (prompt is null)
+        {
+            throw new ArgumentNullException(nameof(prompt));
+        }
+
+        return prompt.ShowNullable(console);
+    }
+
     /// <summary>
     /// Displays a prompt to the user.
     /// </summary>
@@ -34,6 +51,20 @@ public static T Ask<T>(this IAnsiConsole console, string prompt)
         return new TextPrompt<T>(prompt).Show(console);
     }
 
+    /// <summary>
+    /// Displays a prompt to the user.
+    /// </summary>
+    /// <typeparam name="T">The prompt result type.</typeparam>
+    /// <param name="console">The console.</param>
+    /// <param name="prompt">The prompt markup text.</param>
+    /// <returns>The prompt input result.</returns>
+    public static T? AskNullable<T>(this IAnsiConsole console, string prompt)
+    {
+        return new TextPrompt<T>(prompt)
+            .AllowEmpty()
+            .ShowNullable(console);
+    }
+
     /// <summary>
     /// Displays a prompt to the user.
     /// </summary>
@@ -49,6 +80,22 @@ public static T Ask<T>(this IAnsiConsole console, string prompt, CultureInfo? cu
         return textPrompt.Show(console);
     }
 
+    /// <summary>
+    /// Displays a prompt to the user.
+    /// </summary>
+    /// <typeparam name="T">The prompt result type.</typeparam>
+    /// <param name="console">The console.</param>
+    /// <param name="prompt">The prompt markup text.</param>
+    /// <param name="culture">Specific CultureInfo to use when converting input.</param>
+    /// <returns>The prompt input result.</returns>
+    public static T? AskNullable<T>(this IAnsiConsole console, string prompt, CultureInfo? culture)
+    {
+        var textPrompt = new TextPrompt<T>(prompt);
+        textPrompt.AllowEmpty();
+        textPrompt.Culture = culture;
+        return textPrompt.ShowNullable(console);
+    }
+
     /// <summary>
     /// Displays a prompt with two choices, yes or no.
     /// </summary>

src/Spectre.Console/Prompts/INullablePrompt.cs

@@ -0,0 +1,23 @@
+namespace Spectre.Console;
+
+/// <summary>
+/// Represents a prompt.
+/// </summary>
+/// <typeparam name="T">The prompt result type.</typeparam>
+public interface INullablePrompt<T>
+{
+    /// <summary>
+    /// Shows the prompt.
+    /// </summary>
+    /// <param name="console">The console.</param>
+    /// <returns>The prompt input result.</returns>
+    T? ShowNullable(IAnsiConsole console);
+
+    /// <summary>
+    /// Shows the prompt asynchronously.
+    /// </summary>
+    /// <param name="console">The console.</param>
+    /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+    /// <returns>The prompt input result.</returns>
+    Task<T?> ShowNullableAsync(IAnsiConsole console, CancellationToken cancellationToken);
+}

src/Spectre.Console/Prompts/TextPrompt.cs

@@ -4,7 +4,7 @@ namespace Spectre.Console;
 /// Represents a prompt.
 /// </summary>
 /// <typeparam name="T">The prompt result type.</typeparam>
-public sealed class TextPrompt<T> : IPrompt<T>, IHasCulture
+public sealed class TextPrompt<T> : IPrompt<T>, INullablePrompt<T>, IHasCulture
 {
     private readonly string _prompt;
     private readonly StringComparer? _comparer;
@@ -72,7 +72,7 @@ public sealed class TextPrompt<T> : IPrompt<T>, IHasCulture
     /// <summary>
     /// Gets or sets the validator.
     /// </summary>
-    public Func<T, ValidationResult>? Validator { get; set; }
+    public Func<T?, ValidationResult>? Validator { get; set; }
 
     /// <summary>
     /// Gets or sets the style in which the default value is displayed.
@@ -111,8 +111,31 @@ public T Show(IAnsiConsole console)
         return ShowAsync(console, CancellationToken.None).GetAwaiter().GetResult();
     }
 
+    /// <summary>
+    /// Shows the prompt and requests input from the user.
+    /// </summary>
+    /// <param name="console">The console to show the prompt in.</param>
+    /// <returns>The user input converted to the expected type.</returns>
+    /// <inheritdoc/>
+    public T? ShowNullable(IAnsiConsole console)
+    {
+        return ShowNullableAsync(console, CancellationToken.None).GetAwaiter().GetResult();
+    }
+
     /// <inheritdoc/>
     public async Task<T> ShowAsync(IAnsiConsole console, CancellationToken cancellationToken)
+    {
+        return (await ShowNullableAsync(console, false, cancellationToken))
+            ?? throw new Exception("This method do not allow for null");
+    }
+
+    /// <inheritdoc/>
+    public Task<T?> ShowNullableAsync(IAnsiConsole console, CancellationToken cancellationToken)
+    {
+        return ShowNullableAsync(console, true, cancellationToken);
+    }
+
+    private async Task<T?> ShowNullableAsync(IAnsiConsole console, bool allowNull, CancellationToken cancellationToken)
     {
         if (console is null)
         {
@@ -165,7 +188,7 @@ public async Task<T> ShowAsync(IAnsiConsole console, CancellationToken cancellat
                         continue;
                     }
                 }
-                else if (!TypeConverterHelper.TryConvertFromStringWithCulture<T>(input, Culture, out result) || result == null)
+                else if (!TypeConverterHelper.TryConvertFromStringWithCulture<T>(input, Culture, out result) || (!allowNull && result == null))
                 {
                     console.MarkupLine(ValidationErrorMessage);
                     WritePrompt(console);
@@ -232,7 +255,7 @@ private void WritePrompt(IAnsiConsole console)
         console.Markup(markup + " ");
     }
 
-    private bool ValidateResult(T value, [NotNullWhen(false)] out string? message)
+    private bool ValidateResult(T? value, [NotNullWhen(false)] out string? message)
     {
         if (Validator != null)
         {

src/Spectre.Console/Prompts/TextPromptExtensions.cs

@@ -187,7 +187,7 @@ public static TextPrompt<T> DefaultValue<T>(this TextPrompt<T> obj, T value)
     /// <param name="validator">The validation criteria.</param>
     /// <param name="message">The validation error message.</param>
     /// <returns>The same instance so that multiple calls can be chained.</returns>
-    public static TextPrompt<T> Validate<T>(this TextPrompt<T> obj, Func<T, bool> validator, string? message = null)
+    public static TextPrompt<T> Validate<T>(this TextPrompt<T> obj, Func<T?, bool> validator, string? message = null)
     {
         if (obj is null)
         {
@@ -214,7 +214,7 @@ public static TextPrompt<T> Validate<T>(this TextPrompt<T> obj, Func<T, bool> va
     /// <param name="obj">The prompt.</param>
     /// <param name="validator">The validation criteria.</param>
     /// <returns>The same instance so that multiple calls can be chained.</returns>
-    public static TextPrompt<T> Validate<T>(this TextPrompt<T> obj, Func<T, ValidationResult> validator)
+    public static TextPrompt<T> Validate<T>(this TextPrompt<T> obj, Func<T?, ValidationResult> validator)
     {
         if (obj is null)
         {

test/Spectre.Console.Tests/Unit/AnsiConsoleTests.Prompt.cs

@@ -50,5 +50,19 @@ public void Should_Return_Correct_DateTime_When_Asked_US_Culture()
             // Then
             dateTime.ShouldBe(new DateTime(1998, 2, 1));
         }
+
+        [Fact]
+        public void Should_Return_Null_Value_When_Asked_Nullable_US_Culture()
+        {
+            // Given
+            var console = new TestConsole().EmitAnsiSequences();
+            console.Input.PushKey(ConsoleKey.Enter);
+
+            // When
+            var dateTime = console.AskNullable<decimal?>(string.Empty, CultureInfo.GetCultureInfo("en-US"));
+
+            // Then
+            dateTime.ShouldBe(null);
+        }
     }
 }