Greater than 1 FromBody analyzer.

src/Framework/AspNetCoreAnalyzers/src/Analyzers/DiagnosticDescriptors.cs

@@ -196,4 +196,13 @@ internal static class DiagnosticDescriptors
         DiagnosticSeverity.Warning,
         isEnabledByDefault: true,
         helpLinkUri: "https://aka.ms/aspnet/analyzers");
+
+    internal static readonly DiagnosticDescriptor AtMostOneFromBodyAttribute = new(
+        "ASP0024",
+        new LocalizableResourceString(nameof(Resources.Analyzer_MultipleFromBody_Title), Resources.ResourceManager, typeof(Resources)),
+        new LocalizableResourceString(nameof(Resources.Analyzer_MultipleFromBody_Message), Resources.ResourceManager, typeof(Resources)),
+        "Usage",
+        DiagnosticSeverity.Error,
+        isEnabledByDefault: true,
+        helpLinkUri: "https://aka.ms/aspnet/analyzers");
 }

src/Framework/AspNetCoreAnalyzers/src/Analyzers/Resources.resx

@@ -207,4 +207,10 @@
   <data name="Analyzer_HeaderDictionaryAdd_Title" xml:space="preserve">
     <value>Suggest using IHeaderDictionary.Append or the indexer</value>
   </data>
-</root>
+  <data name="Analyzer_MultipleFromBody_Message" xml:space="preserve">
+    <value>Route handler has multiple parameters with the [FromBody] attribute or a parameter with an [AsParameters] attribute where the parameter type contains multiple members with [FromBody] attributes. Only one parameter can have a [FromBody] attribute.</value>
+  </data>
+  <data name="Analyzer_MultipleFromBody_Title" xml:space="preserve">
+    <value>Route handler has multiple parameters with the [FromBody] attribute.</value>
+  </data>
+</root>

src/Framework/AspNetCoreAnalyzers/src/Analyzers/RouteHandlers/AtMostOneFromBodyAttribute.cs

@@ -0,0 +1,62 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNetCore.Analyzers.RouteEmbeddedLanguage.Infrastructure;
+using Microsoft.AspNetCore.App.Analyzers.Infrastructure;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace Microsoft.AspNetCore.Analyzers.RouteHandlers;
+
+using WellKnownType = WellKnownTypeData.WellKnownType;
+
+public partial class RouteHandlerAnalyzer : DiagnosticAnalyzer
+{
+    private static void AtMostOneFromBodyAttribute(
+        in OperationAnalysisContext context,
+        WellKnownTypes wellKnownTypes,
+        IMethodSymbol methodSymbol)
+    {
+        var fromBodyMetadataInterfaceType = wellKnownTypes.Get(WellKnownType.Microsoft_AspNetCore_Http_Metadata_IFromBodyMetadata);
+        var asParametersAttributeType = wellKnownTypes.Get(WellKnownType.Microsoft_AspNetCore_Http_AsParametersAttribute);
+
+        var asParametersDecoratedParameters = methodSymbol.Parameters.Where(p => p.HasAttribute(asParametersAttributeType));
+
+        foreach (var asParameterDecoratedParameter in asParametersDecoratedParameters)
+        {
+            var fromBodyMetadataInterfaceMembers = asParameterDecoratedParameter.Type.GetMembers().Where(
+                m => m.HasAttributeImplementingInterface(fromBodyMetadataInterfaceType)
+                );
+
+            if (fromBodyMetadataInterfaceMembers.Count() >= 2)
+            {
+                ReportDiagnostics(context, fromBodyMetadataInterfaceMembers);
+            }
+        }
+
+        var fromBodyMetadataInterfaceParameters = methodSymbol.Parameters.Where(p => p.HasAttributeImplementingInterface(fromBodyMetadataInterfaceType));
+
+        if (fromBodyMetadataInterfaceParameters.Count() >= 2)
+        {
+            ReportDiagnostics(context, fromBodyMetadataInterfaceParameters);
+        }
+
+        static void ReportDiagnostics(OperationAnalysisContext context, IEnumerable<ISymbol> symbols)
+        {
+            foreach (var symbol in symbols)
+            {
+                if (symbol.DeclaringSyntaxReferences.Length > 0)
+                {
+                    var syntax = symbol.DeclaringSyntaxReferences[0].GetSyntax(context.CancellationToken);
+                    var location = syntax.GetLocation();
+                    context.ReportDiagnostic(Diagnostic.Create(
+                        DiagnosticDescriptors.AtMostOneFromBodyAttribute,
+                        location
+                        ));
+                }
+            }
+        }
+    }
+}

src/Framework/AspNetCoreAnalyzers/src/Analyzers/RouteHandlers/DisallowNonParsableComplexTypesOnParameters.cs

@@ -17,11 +17,10 @@ public partial class RouteHandlerAnalyzer : DiagnosticAnalyzer
 {
     private static void DisallowNonParsableComplexTypesOnParameters(
         in OperationAnalysisContext context,
+        WellKnownTypes wellKnownTypes,
         RouteUsageModel routeUsage,
         IMethodSymbol methodSymbol)
     {
-        var wellKnownTypes = WellKnownTypes.GetOrCreate(context.Compilation);
-
         foreach (var handlerDelegateParameter in methodSymbol.Parameters)
         {
             // If the parameter is decorated with a FromServices attribute then we can skip it.

src/Framework/AspNetCoreAnalyzers/src/Analyzers/RouteHandlers/RouteHandlerAnalyzer.cs

@@ -27,7 +27,8 @@ public partial class RouteHandlerAnalyzer : DiagnosticAnalyzer
         DiagnosticDescriptors.DetectMismatchedParameterOptionality,
         DiagnosticDescriptors.RouteParameterComplexTypeIsNotParsableOrBindable,
         DiagnosticDescriptors.BindAsyncSignatureMustReturnValueTaskOfT,
-        DiagnosticDescriptors.AmbiguousRouteHandlerRoute
+        DiagnosticDescriptors.AmbiguousRouteHandlerRoute,
+        DiagnosticDescriptors.AtMostOneFromBodyAttribute
     );
 
     public override void Initialize(AnalysisContext context)
@@ -105,17 +106,19 @@ void DoOperationAnalysis(OperationAnalysisContext context, ConcurrentDictionary<
                 {
                     var lambda = (IAnonymousFunctionOperation)delegateCreation.Target;
                     DisallowMvcBindArgumentsOnParameters(in context, wellKnownTypes, invocation, lambda.Symbol);
-                    DisallowNonParsableComplexTypesOnParameters(in context, routeUsage, lambda.Symbol);
+                    DisallowNonParsableComplexTypesOnParameters(in context, wellKnownTypes, routeUsage, lambda.Symbol);
                     DisallowReturningActionResultFromMapMethods(in context, wellKnownTypes, invocation, lambda, delegateCreation.Syntax);
                     DetectMisplacedLambdaAttribute(context, lambda);
                     DetectMismatchedParameterOptionality(in context, routeUsage, lambda.Symbol);
+                    AtMostOneFromBodyAttribute(in context, wellKnownTypes, lambda.Symbol);
                 }
                 else if (delegateCreation.Target.Kind == OperationKind.MethodReference)
                 {
                     var methodReference = (IMethodReferenceOperation)delegateCreation.Target;
                     DisallowMvcBindArgumentsOnParameters(in context, wellKnownTypes, invocation, methodReference.Method);
-                    DisallowNonParsableComplexTypesOnParameters(in context, routeUsage, methodReference.Method);
+                    DisallowNonParsableComplexTypesOnParameters(in context, wellKnownTypes, routeUsage, methodReference.Method);
                     DetectMismatchedParameterOptionality(in context, routeUsage, methodReference.Method);
+                    AtMostOneFromBodyAttribute(in context, wellKnownTypes, methodReference.Method);
 
                     var foundMethodReferenceBody = false;
                     if (!methodReference.Method.DeclaringSyntaxReferences.IsEmpty)

src/Framework/AspNetCoreAnalyzers/test/RouteHandlers/AtMostOneFromBodyAttributeTest.cs

@@ -0,0 +1,173 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Security.Policy;
+using Microsoft.CodeAnalysis.Testing;
+using VerifyCS = Microsoft.AspNetCore.Analyzers.Verifiers.CSharpAnalyzerVerifier<Microsoft.AspNetCore.Analyzers.RouteHandlers.RouteHandlerAnalyzer>;
+
+namespace Microsoft.AspNetCore.Analyzers.RouteHandlers;
+
+public partial class AtMostOneFromBodyAttributeTest
+{
+    private TestDiagnosticAnalyzerRunner Runner { get; } = new(new RouteHandlerAnalyzer());
+
+    [Fact]
+    public async Task Handler_With_No_FromBody_Attributes_Works()
+    {
+        // Arrange
+        var source = @"
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.MapPost(""/products/{productId}"", (string productId, Product product) => {});
+
+public class Product
+{
+}
+";
+
+        // Act
+        await VerifyCS.VerifyAnalyzerAsync(source);
+    }
+
+    [Fact]
+    public async Task Handler_With_One_FromBody_Attributes_Works()
+    {
+        // Arrange
+        var source = @"
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.MapPost(""/products/{productId}"", (string productId, [FromBody]Product product) => {});
+
+public class Product
+{
+}
+";
+
+        // Act
+        await VerifyCS.VerifyAnalyzerAsync(source);
+    }
+
+    [Fact]
+    public async Task Handler_With_Two_FromBody_Attributes_Fails()
+    {
+        // Arrange
+        var source = @"
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.MapPost(""/products/{productId}"", (string productId, {|#0:[FromBody]Product product1|}, {|#1:[FromBody]Product product2|}) => {});
+
+public class Product
+{
+}
+";
+
+        var expectedDiagnostic1 = new DiagnosticResult(DiagnosticDescriptors.AtMostOneFromBodyAttribute).WithLocation(0);
+        var expectedDiagnostic2 = new DiagnosticResult(DiagnosticDescriptors.AtMostOneFromBodyAttribute).WithLocation(1);
+
+        // Act
+        await VerifyCS.VerifyAnalyzerAsync(
+            source,
+            expectedDiagnostic1,
+            expectedDiagnostic2
+            );
+    }
+
+    [Fact]
+    public async Task MethodGroup_Handler_With_Two_FromBody_Attributes_Fails()
+    {
+        // Arrange
+        var source = @"
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.MapPost(""/products/{productId}"", MyHandlers.ProcessRequest);
+
+public static class MyHandlers
+{
+    public static void ProcessRequest(string productId, {|#0:[FromBody]Product product1|}, {|#1:[FromBody]Product product2|})
+    {
+    }
+}
+
+public class Product
+{
+}
+";
+
+        var expectedDiagnostic1 = new DiagnosticResult(DiagnosticDescriptors.AtMostOneFromBodyAttribute).WithLocation(0);
+        var expectedDiagnostic2 = new DiagnosticResult(DiagnosticDescriptors.AtMostOneFromBodyAttribute).WithLocation(1);
+
+        // Act
+        await VerifyCS.VerifyAnalyzerAsync(
+            source,
+            expectedDiagnostic1,
+            expectedDiagnostic2
+            );
+    }
+
+    [Fact]
+    public async Task Handler_Handler_With_AsParameters_Argument_With_TwoFromBody_Attributes_Fails()
+    {
+        // Arrange
+        var source = @"
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.MapPost(""/products/{productId}"", ([AsParameters]GetProductRequest request) => {});
+
+public class GetProductRequest
+{
+    {|#0:[FromBody]
+    public Product Product1 { get; set; }|}
+
+    {|#1:[FromBody]
+    public Product Product2 { get; set; }|}
+}
+
+public class Product
+{
+}
+";
+
+        var expectedDiagnostic1 = new DiagnosticResult(DiagnosticDescriptors.AtMostOneFromBodyAttribute).WithLocation(0);
+        var expectedDiagnostic2 = new DiagnosticResult(DiagnosticDescriptors.AtMostOneFromBodyAttribute).WithLocation(1);
+
+        // Act
+        await VerifyCS.VerifyAnalyzerAsync(
+            source,
+            expectedDiagnostic1,
+            expectedDiagnostic2
+            );
+    }
+
+    [Fact]
+    public async Task Handler_Handler_With_AsParameters_Argument_With_OneFromBody_Attributes_Works()
+    {
+        // Arrange
+        var source = @"
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Builder;
+var webApp = WebApplication.Create();
+webApp.MapPost(""/products/{productId}"", ([AsParameters]GetProductRequest request) => {});
+
+public class GetProductRequest
+{
+    {|#0:[FromBody]
+    public Product Product1 { get; set; }|}
+
+    public Product Product2 { get; set; }
+}
+
+public class Product
+{
+}
+";
+
+        // Act
+        await VerifyCS.VerifyAnalyzerAsync(source);
+    }
+}