Treat empty query strings as null for nullable value types

src/Http/Http.Extensions/src/RequestDelegateFactory.cs

@@ -1820,9 +1820,16 @@ private static Expression BindParameterFromValue(ParameterInfo parameter, Expres
                     Expression.Assign(parameter.ParameterType.IsArray ? Expression.ArrayAccess(argument, index) : argument, Expression.Convert(parsedValue, targetParseType)),
                     failBlock));
 
+        // For nullable value types, treat empty strings as null (don't attempt to parse them).
+        // This matches controller behavior and avoids parse failures on ?param= (empty value).
+        var isNullableValueType = Nullable.GetUnderlyingType(parameter.ParameterType) != null;
+        var sourceCheckExpr = (Expression)(isNullableValueType
+            ? TempSourceStringIsNotNullOrEmptyExpr
+            : TempSourceStringNotNullExpr);
+
         var ifNotNullTryParse = !parameter.HasDefaultValue
-            ? Expression.IfThen(TempSourceStringNotNullExpr, tryParseExpression)
-            : Expression.IfThenElse(TempSourceStringNotNullExpr, tryParseExpression,
+            ? Expression.IfThen(sourceCheckExpr, tryParseExpression)
+            : Expression.IfThenElse(sourceCheckExpr, tryParseExpression,
                 Expression.Assign(argument,
                 CreateDefaultValueExpression(parameter.DefaultValue, parameter.ParameterType)));
 

src/Http/Http.Extensions/test/RequestDelegateFactoryTests.cs

@@ -263,6 +263,90 @@ public async Task NullRouteParametersPrefersRouteOverQueryString()
         Assert.Equal(42, httpContext.Items["input"]);
     }
 
+    [Fact]
+    public async Task EmptyQueryStringValueTreatedAsNullForNullableBool()
+    {
+        var httpContext = CreateHttpContext();
+
+        var factoryResult = RequestDelegateFactory.Create((bool? value, HttpContext httpContext) =>
+        {
+            httpContext.Items["value"] = value;
+        });
+
+        httpContext.Request.Query = new QueryCollection(new Dictionary<string, StringValues>
+        {
+            ["value"] = ""  // Empty string should be treated as null
+        });
+
+        var requestDelegate = factoryResult.RequestDelegate;
+        await requestDelegate(httpContext);
+
+        Assert.Null(httpContext.Items["value"]);
+    }
+
+    [Fact] 
+    public async Task EmptyQueryStringValueTreatedAsNullForNullableInt()
+    {
+        var httpContext = CreateHttpContext();
+
+        var factoryResult = RequestDelegateFactory.Create((int? value, HttpContext httpContext) =>
+        {
+            httpContext.Items["value"] = value;
+        });
+
+        httpContext.Request.Query = new QueryCollection(new Dictionary<string, StringValues>
+        {
+            ["value"] = ""  // Empty string should be treated as null
+        });
+
+        var requestDelegate = factoryResult.RequestDelegate;
+        await requestDelegate(httpContext);
+
+        Assert.Null(httpContext.Items["value"]);
+    }
+
+    [Fact]
+    public async Task NonEmptyQueryStringValueParsedCorrectlyForNullableBool()
+    {
+        var httpContext = CreateHttpContext();
+
+        var factoryResult = RequestDelegateFactory.Create((bool? value, HttpContext httpContext) =>
+        {
+            httpContext.Items["value"] = value;
+        });
+
+        httpContext.Request.Query = new QueryCollection(new Dictionary<string, StringValues>
+        {
+            ["value"] = "true"
+        });
+
+        var requestDelegate = factoryResult.RequestDelegate;
+        await requestDelegate(httpContext);
+
+        Assert.True((bool?)httpContext.Items["value"]);
+    }
+
+    [Fact]
+    public async Task NonEmptyQueryStringValueParsedCorrectlyForNullableInt()
+    {
+        var httpContext = CreateHttpContext();
+
+        var factoryResult = RequestDelegateFactory.Create((int? value, HttpContext httpContext) =>
+        {
+            httpContext.Items["value"] = value;
+        });
+
+        httpContext.Request.Query = new QueryCollection(new Dictionary<string, StringValues>
+        {
+            ["value"] = "42"
+        });
+
+        var requestDelegate = factoryResult.RequestDelegate;
+        await requestDelegate(httpContext);
+
+        Assert.Equal(42, (int?)httpContext.Items["value"]);
+    }
+
     [Fact]
     public async Task CreatingDelegateWithInstanceMethodInfoCreatesInstancePerCall()
     {

src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.QueryParameters.cs

@@ -226,4 +226,44 @@ public async Task MapAction_ExplicitQueryParam_NameTest(string name, string look
         await endpoint.RequestDelegate(httpContext);
         await VerifyResponseBodyAsync(httpContext, "test", 200);
     }
+
+    [Fact]
+    public async Task MapAction_NullableBoolParam_WithEmptyQueryStringValue_SetsNull()
+    {
+        // Regression test for https://github.com/dotnet/aspnetcore/issues/65754
+        // ?myBool= should bind to null for bool? parameter, not throw.
+        var (results, compilation) = await RunGeneratorAsync("""
+app.MapGet("/test", ([FromQuery] bool? myBool) => myBool.HasValue ? $"Value: {myBool.Value}" : "null");
+""");
+        var endpoint = GetEndpointFromCompilation(compilation);
+
+        VerifyStaticEndpointModel(results, endpointModel =>
+        {
+            Assert.Equal("MapGet", endpointModel.HttpMethod);
+            var p = Assert.Single(endpointModel.Parameters);
+            Assert.Equal(EndpointParameterSource.Query, p.Source);
+            Assert.Equal("myBool", p.SymbolName);
+        });
+
+        var httpContext = CreateHttpContext();
+        httpContext.Request.QueryString = new QueryString("?myBool=");
+
+        await endpoint.RequestDelegate(httpContext);
+        await VerifyResponseBodyAsync(httpContext, "null");
+    }
+
+    [Fact]
+    public async Task MapAction_NullableIntParam_WithEmptyQueryStringValue_SetsNull()
+    {
+        var (results, compilation) = await RunGeneratorAsync("""
+app.MapGet("/test", ([FromQuery] int? value) => value.HasValue ? $"{value.Value}" : "null");
+""");
+        var endpoint = GetEndpointFromCompilation(compilation);
+
+        var httpContext = CreateHttpContext();
+        httpContext.Request.QueryString = new QueryString("?value=");
+
+        await endpoint.RequestDelegate(httpContext);
+        await VerifyResponseBodyAsync(httpContext, "null");
+    }
 }