将路由查询参数绑定到默认属性而不在 URL 中指定它

Question

I'm creating a .NET Core 3 WebApi and i'm having some 'troubles' with modelbinding from the query parameters. I have a Range class with Min , Max end Value properties. The usage of this class is to filter with a range or a constant value.

    public class Range
    {
        public int? Min { get; set; }
        public int? Max { get; set; }
        public int? Value { get; set; }
        public static implicit operator Range(int value) => new Range {Value = value};
    }

When i don't define a sub property i want it to bind to the Value property, like api/contacts?age=40 .
The Min and Max works as planned with the default binding way like api/contacts?age.min=18&age.max=30 . I thought adding an implicit operator would work but it doesn't.

Is there a way (like an attribute) to make Value as default property?

Answer 1

Implicit operator does an implicit conversion when you assign an 'int' value to a 'Range' object. However, it could not be applied in model binding scenario of ASP.NET Core. The model binding only checks the data from request and call the model binder to bind data for different types of classes.

The asp.net core framework also provide 'TypeConverter' to determine whether it should convert a string to an object. However, it could only deal with one query string and disturb a common model binding process. (eg age.max and age.min).

For your problem, the only way I suggest is to construct a custom model binder so that it will bind data following the custom rules: (Make your case as an example)

The steps of the custom model binding for complex model as below:

1. Create a custom binder, for example, RangeEntityBinder which should extend IModelBinder

2. Create a custom binder provider, for example, RangeEntityBinderProvider which should extend IModelBinderProvider

3. Register the binder provider into ModelBinderProviders in ConfigureServices of Startup file

According to your description, I construct a demo which you could refer to.

Controller:

public IActionResult RangePage(Range age) 
        {
            if(age == null)
            {
                age = new Range();
            }
            return View(age);
        }

RangePage.cshtml:

<a asp-controller="Home" asp-action="RangePage" asp-route-age.min="18" asp-route-age.max="30">age.min=18&age.max=30</a>
<br />
<a asp-controller="Home" asp-action="RangePage" asp-route-age="40">age=40</a>
<div class="container">
    <label asp-for="Max">Max: </label>@Model.Max
    <br />
    <label asp-for="Min">Min: </label>@Model.Min
    <br />
    <label asp-for="Value">Value: </label>@Model.Value
</div>

Range.cs

public class Range
{
    public int? Min { get; set; }
    public int? Max { get; set; }

    public int? Value { get; set; }

}

RangeEntityBinder.cs

public class RangeEntityBinder : IModelBinder
    {
        private readonly ComplexTypeModelBinder worker;


        public RangeEntityBinder(Dictionary<ModelMetadata, IModelBinder> propertyBinders, ILoggerFactory loggerFactory)
        {
            worker = new ComplexTypeModelBinder(propertyBinders, loggerFactory);
        }

        public async Task BindModelAsync(ModelBindingContext bindingContext)
        {
            if (bindingContext == null)
            {
                throw new ArgumentNullException(nameof(bindingContext));
            }

            // Try get the "age" to populate the model
            var modelName = bindingContext.ModelName;

            // Try to fetch the value of the argument by name
            var valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);

            // If find 'age', return Range object with Value="age"
            // If not, use ComplexTypeModelBinder do the common binding
            if (valueProviderResult == ValueProviderResult.None)
            {
                await this.worker.BindModelAsync(bindingContext);
                if (!bindingContext.Result.IsModelSet)
                {
                    return;
                }

                var model= bindingContext.Result.Model as Range;
                if (model== null)
                {
                    throw new InvalidOperationException($"Expected {bindingContext.ModelName} to have been bound by ComplexTypeModelBinder");
                }
            }
            else
            {
                var value = valueProviderResult.FirstValue;

                // Check if the argument value is null or empty
                if (string.IsNullOrEmpty(value))
                {
                    await Task.CompletedTask;
                }

                if (!int.TryParse(value, out var ageValue))
                {
                    // Non-integer arguments result in model state errors
                    bindingContext.ModelState.TryAddModelError(
                        modelName, "Age value must be an integer.");

                    await Task.CompletedTask;
                    return;
                }

                var model = new Range()
                {
                    Value = ageValue
                };

                bindingContext.Result = ModelBindingResult.Success(model);
                await Task.CompletedTask;
            }


        }
    }

RangeEntityBinderProvider.cs

public class RangeEntityBinderProvider: IModelBinderProvider
    {

        public IModelBinder GetBinder(ModelBinderProviderContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException(nameof(context));
            }

            // If type is Range, use RangeEntityBinder to bind model
            if (context.Metadata.ModelType == typeof(Range))
            {
                var propertyBinders = new Dictionary<ModelMetadata, IModelBinder>();
                for (var i = 0; i < context.Metadata.Properties.Count; i++)
                {
                    var property = context.Metadata.Properties[i];
                    propertyBinders.Add(property, context.CreateBinder(property));
                }

                var loggerFactory = context.Services.GetRequiredService<ILoggerFactory>();

                return new RangeEntityBinder(propertyBinders, loggerFactory);
            }

            return null;
        }
    }

ConfigureServices method

public void ConfigureServices(IServiceCollection services)
        {
            /* Other services*/

            services.AddMvc(options =>
            {
                options.ModelBinderProviders.Insert(0, new RangeEntityBinderProvider());
            });

            /* Other services*/

        }

Demo: