Add custom ModelBinder to ensure query parameters of type string are bound as empty strings instead of null (#1098)

Author: claudiamurialdo

dotnet/src/dotnetcore/GxClasses.Web/Middleware/GXRestServices.cs

@@ -14,6 +14,9 @@
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.Filters;
+using Microsoft.AspNetCore.Mvc.ModelBinding.Binders;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using System.Threading.Tasks;
 
 
 namespace GeneXus.Utils
@@ -29,6 +32,36 @@ public void OnActionExecuting(ActionExecutingContext context)
 			(context.Controller as GxRestService).Initialize();
 		}
 	}
+
+	public class QueryStringModelBinderProvider : IModelBinderProvider
+	{
+		public IModelBinder GetBinder(ModelBinderProviderContext context)
+		{
+			if (context.BindingInfo.BindingSource == BindingSource.Query &&
+				context.Metadata.ModelType == typeof(string))
+			{
+				return new BinderTypeModelBinder(typeof(CustomQueryStringBinder));
+			}
+			return null;
+		}
+	}
+	public class CustomQueryStringBinder : IModelBinder
+	{
+		public Task BindModelAsync(ModelBindingContext bindingContext)
+		{
+			if (!bindingContext.BindingSource.CanAcceptDataFrom(BindingSource.Query))
+			{
+				return Task.CompletedTask;
+			}
+			if (bindingContext.ModelType != typeof(string))
+			{
+				return Task.CompletedTask;
+			}
+			string value = bindingContext.ValueProvider.GetValue(bindingContext.ModelName).FirstValue;
+			bindingContext.Result = ModelBindingResult.Success(value ?? string.Empty);
+			return Task.CompletedTask;
+		}
+	}
 	[TypeFilter(typeof(CustomActionFilter))]
 	public class GxRestService : ControllerBase
 	{

dotnet/src/dotnetcore/GxNetCoreStartup/Startup.cs

@@ -268,9 +268,13 @@ public void ConfigureServices(IServiceCollection services)
 		private void RegisterControllerAssemblies(IMvcBuilder mvcBuilder)
 		{
 
-			if (RestAPIHelpers.ServiceAsController() && !string.IsNullOrEmpty(VirtualPath))
+			if (RestAPIHelpers.ServiceAsController())
 			{
-				mvcBuilder.AddMvcOptions(options =>	options.Conventions.Add(new SetRoutePrefix(new RouteAttribute(VirtualPath))));
+				mvcBuilder.AddMvcOptions(options => options.ModelBinderProviders.Insert(0, new QueryStringModelBinderProvider()));
+				if (!string.IsNullOrEmpty(VirtualPath))
+				{
+					mvcBuilder.AddMvcOptions(options => options.Conventions.Add(new SetRoutePrefix(new RouteAttribute(VirtualPath))));
+				}
 			}
 
 			if (RestAPIHelpers.JsonSerializerCaseSensitive())