Reuse model binders (#22391)

* Reuse model binders

Reuse the same model binders for CancellationTokens and services, rather than allocating a new one for every request with such a parameter.

Author: martincostello

src/Mvc/Mvc.Core/src/ModelBinding/Binders/CancellationTokenModelBinderProvider.cs

@@ -11,6 +11,10 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
     /// </summary>
     public class CancellationTokenModelBinderProvider : IModelBinderProvider
     {
+        // CancellationTokenModelBinder does not have any state. Re-use the same instance for binding.
+
+        private readonly CancellationTokenModelBinder _modelBinder = new CancellationTokenModelBinder();
+
         /// <inheritdoc />
         public IModelBinder GetBinder(ModelBinderProviderContext context)
         {
@@ -21,7 +25,7 @@ public IModelBinder GetBinder(ModelBinderProviderContext context)
 
             if (context.Metadata.ModelType == typeof(CancellationToken))
             {
-                return new CancellationTokenModelBinder();
+                return _modelBinder;
             }
 
             return null;

src/Mvc/Mvc.Core/src/ModelBinding/Binders/ServicesModelBinderProvider.cs

@@ -10,6 +10,10 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
     /// </summary>
     public class ServicesModelBinderProvider : IModelBinderProvider
     {
+        // ServicesModelBinder does not have any state. Re-use the same instance for binding.
+
+        private readonly ServicesModelBinder _modelBinder = new ServicesModelBinder();
+
         /// <inheritdoc />
         public IModelBinder GetBinder(ModelBinderProviderContext context)
         {
@@ -21,7 +25,7 @@ public IModelBinder GetBinder(ModelBinderProviderContext context)
             if (context.BindingInfo.BindingSource != null &&
                 context.BindingInfo.BindingSource.CanAcceptDataFrom(BindingSource.Services))
             {
-                return new ServicesModelBinder();
+                return _modelBinder;
             }
 
             return null;