Refine parameter normalization to only apply to default configuration keys

Co-authored-by: davidfowl <95136+davidfowl@users.noreply.github.com>

Author: Copilot

src/Aspire.Hosting/ParameterResourceBuilderExtensions.cs

@@ -203,14 +203,15 @@ public static IResourceBuilder<ParameterResource> WithCustomInput(this IResource
 
     private static string GetParameterValue(ConfigurationManager configuration, string name, ParameterDefault? parameterDefault, string? configurationKey = null)
     {
+        var useDefaultKey = configurationKey is null;
         configurationKey ??= $"Parameters:{name}";
 
         // First try to get the value with the exact configuration key
         var value = configuration[configurationKey];
 
-        // If not found and the name contains dashes, try with underscores as a fallback
+        // If not found, the default key was used, and the name contains dashes, try with underscores as a fallback
         // This supports command-line arguments and environment variables where dashes are replaced with underscores
-        if (value is null && name.Contains('-', StringComparison.Ordinal))
+        if (value is null && useDefaultKey && name.Contains('-', StringComparison.Ordinal))
         {
             var normalizedKey = $"Parameters:{name.Replace("-", "_", StringComparison.Ordinal)}";
             value = configuration[normalizedKey];

tests/Aspire.Hosting.Tests/AddParameterTests.cs

@@ -579,4 +579,29 @@ public void ParameterWithoutDash_DoesNotFallbackToUnderscore()
 #pragma warning restore CS0618 // Type or member is obsolete
         });
     }
+
+    [Fact]
+    public void ParameterWithCustomConfigurationKey_DoesNotUseFallback()
+    {
+        // Arrange
+        var appBuilder = DistributedApplication.CreateBuilder();
+        
+        // Set configuration with custom key that has underscore
+        appBuilder.Configuration.AddInMemoryCollection(new Dictionary<string, string?>
+        {
+            ["CustomSection:my_key"] = "custom-value"
+        });
+
+        // Act - use custom configuration key
+        appBuilder.AddParameterFromConfiguration("my-param", "CustomSection:my-key");
+
+        using var app = appBuilder.Build();
+        var appModel = app.Services.GetRequiredService<DistributedApplicationModel>();
+        var parameterResource = Assert.Single(appModel.Resources.OfType<ParameterResource>());
+
+        // Assert - should find the value using the custom key without any fallback logic
+#pragma warning disable CS0618 // Type or member is obsolete
+        Assert.Equal("custom-value", parameterResource.Value);
+#pragma warning restore CS0618 // Type or member is obsolete
+    }
 }