ASP.NET Core MVC中所提供的Model Binding功能簡單但實用，其主要目的是將請求中包含的數據映射到action的方法參數中。這樣就避免了開發者像在Web Forms時代那樣需要從Request類中手動獲取數據的繁鎖操作，直接提高了開發效率。此功能繼承自ASP.NET MVC，所以熟悉上一代框架開發的工程師，可以毫無障礙地繼續享有它的便利。

本文想要探索下Model Binding相關的內容，這裏先從源碼中找到其發生的時機與場合。

在ControllerActionInvoker類的Next方法內部，可以看到對BindArgumentsAsync方法的調用，這裏即會對方法的參數進行綁定數據的處理。

private Task Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
{
    switch (next)
    {
        case State.ActionBegin:
            {
                var controllerContext = _controllerContext;

                _cursor.Reset();

                _instance = _cacheEntry.ControllerFactory(controllerContext);

                _arguments = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);

                var task = BindArgumentsAsync();
                if (task.Status != TaskStatus.RanToCompletion)
                {
                    next = State.ActionNext;
                    return task;
                }

                goto case State.ActionNext;
            }
        ...
    }
}
 

此方法又調用了ControllerActionInvokerCacheEntry類中ControllerBinderDelegate屬性，該屬性是一個delegate方法，所以傳入參數後可直接執行處理。

private Task BindArgumentsAsync()
{
    ...
    
    return _cacheEntry.ControllerBinderDelegate(_controllerContext, _instance, _arguments);
}

創建ControllerActionInvokerCacheEntry的地方是前兩篇文章(Controller，Action)中已經提到過的ControllerActionInvokerCache類。

public (ControllerActionInvokerCacheEntry cacheEntry, IFilterMetadata[] filters) GetCachedResult(ControllerContext controllerContext)
{
    ...
    if (!cache.Entries.TryGetValue(actionDescriptor, out var cacheEntry))
    {
        ...
        var propertyBinderFactory = ControllerBinderDelegateProvider.CreateBinderDelegate(
            _parameterBinder,
            _modelBinderFactory,
            _modelMetadataProvider,
            actionDescriptor);

        var actionMethodExecutor = ActionMethodExecutor.GetExecutor(objectMethodExecutor);

        cacheEntry = new ControllerActionInvokerCacheEntry(
            filterFactoryResult.CacheableFilters, 
            controllerFactory, 
            controllerReleaser,
            propertyBinderFactory,
            objectMethodExecutor,
            actionMethodExecutor);
        cacheEntry = cache.Entries.GetOrAdd(actionDescriptor, cacheEntry);
    }
    ...

    return (cacheEntry, filters);
}

於是跟蹤至ControllerBinderDelegateProvider類，找到CreateBinderDelegate方法。

public static ControllerBinderDelegate CreateBinderDelegate(
    ParameterBinder parameterBinder,
    IModelBinderFactory modelBinderFactory,
    IModelMetadataProvider modelMetadataProvider,
    ControllerActionDescriptor actionDescriptor)
{
    ...

    var parameterBindingInfo = GetParameterBindingInfo(modelBinderFactory, modelMetadataProvider, actionDescriptor);
    ...

    return Bind;

    async Task Bind(ControllerContext controllerContext, object controller, Dictionary<string, object> arguments)
    {
        var valueProvider = await CompositeValueProvider.CreateAsync(controllerContext);
        var parameters = actionDescriptor.Parameters;

        for (var i = 0; i < parameters.Count; i++)
        {
            var parameter = parameters[i];
            var bindingInfo = parameterBindingInfo[i];
            var modelMetadata = bindingInfo.ModelMetadata;

            if (!modelMetadata.IsBindingAllowed)
            {
                continue;
            }

            var result = await parameterBinder.BindModelAsync(
                controllerContext,
                bindingInfo.ModelBinder,
                valueProvider,
                parameter,
                modelMetadata,
                value: null);

            if (result.IsModelSet)
            {
                arguments[parameter.Name] = result.Model;
            }
        }

        ...
    }
}

這裏可以看到創建綁定的delegate方法，與之對應的是之前那句_cacheEntry.ControllerBinderDelegate(_controllerContext, _instance, _arguments)代碼。

public virtual async Task<ModelBindingResult> BindModelAsync(
    ActionContext actionContext,
    IModelBinder modelBinder,
    IValueProvider valueProvider,
    ParameterDescriptor parameter,
    ModelMetadata metadata,
    object value)
{
    ...

    var modelBindingContext = DefaultModelBindingContext.CreateBindingContext(
        actionContext,
        valueProvider,
        metadata,
        parameter.BindingInfo,
        parameter.Name);
    modelBindingContext.Model = value;

    ...

    await modelBinder.BindModelAsync(modelBindingContext);

    ...

    var modelBindingResult = modelBindingContext.Result;

    ...

    return modelBindingResult;
}

到了此處，就是旅程的終點。ParameterBinder類的BindModelAsync中可以找到對IModelBinder類型的BindModelAsync方法的調用。Model Binding這一操作便是在此時此地實現的。

接下來的疑問有兩處，modelBinder是如何產生的，請求中的數據又是怎樣與modelBinder發生聯系。

ModelBinder

回到ControllerBinderDelegateProvider類的CreateBinderDelegate方法，可以看到其中調用了GetParameterBindingInfo方法。

private static BinderItem[] GetParameterBindingInfo(
    IModelBinderFactory modelBinderFactory,
    IModelMetadataProvider modelMetadataProvider,
    ControllerActionDescriptor actionDescriptor)
{
    var parameters = actionDescriptor.Parameters;
    ...

    var parameterBindingInfo = new BinderItem[parameters.Count];
    for (var i = 0; i < parameters.Count; i++)
    {
        var parameter = parameters[i];

        ...

        var binder = modelBinderFactory.CreateBinder(new ModelBinderFactoryContext
        {
            BindingInfo = parameter.BindingInfo,
            Metadata = metadata,
            CacheToken = parameter,
        });

        parameterBindingInfo[i] = new BinderItem(binder, metadata);
    }

    return parameterBindingInfo;
}

這裏的代碼很明顯地說明了modelBinder由ModelBinderFactory類的CreateBinder方法創建。

public IModelBinder CreateBinder(ModelBinderFactoryContext context)
{
    ...

    IModelBinder binder;
    if (TryGetCachedBinder(context.Metadata, context.CacheToken, out binder))
    {
        return binder;
    }

    var providerContext = new DefaultModelBinderProviderContext(this, context);
    binder = CreateBinderCoreUncached(providerContext, context.CacheToken);
    ...
    AddToCache(context.Metadata, context.CacheToken, binder);

    return binder;
}

CreateBinder方法內部中如果緩存可以取到值，則從緩存內取值並直接返回，否則通過CreateBinderCoreUncached方法取值。

private IModelBinder CreateBinderCoreUncached(DefaultModelBinderProviderContext providerContext, object token)
{
    ...

    IModelBinder result = null;

    for (var i = 0; i < _providers.Length; i++)
    {
        var provider = _providers[i];
        result = provider.GetBinder(providerContext);
        if (result != null)
        {
            break;
        }
    }

    ...

    return result;
}

這裏的providers集合又包含哪些數據呢？可以從MvcCoreMvcOptionsSetup類中找到答案。

public void Configure(MvcOptions options)
{
    // Set up ModelBinding
    options.ModelBinderProviders.Add(new BinderTypeModelBinderProvider());
    options.ModelBinderProviders.Add(new ServicesModelBinderProvider());
    options.ModelBinderProviders.Add(new BodyModelBinderProvider(options.InputFormatters, _readerFactory, _loggerFactory, options));
    options.ModelBinderProviders.Add(new HeaderModelBinderProvider());
    options.ModelBinderProviders.Add(new FloatingPointTypeModelBinderProvider());
    options.ModelBinderProviders.Add(new EnumTypeModelBinderProvider(options));
    options.ModelBinderProviders.Add(new SimpleTypeModelBinderProvider());
    options.ModelBinderProviders.Add(new CancellationTokenModelBinderProvider());
    options.ModelBinderProviders.Add(new ByteArrayModelBinderProvider());
    options.ModelBinderProviders.Add(new FormFileModelBinderProvider());
    options.ModelBinderProviders.Add(new FormCollectionModelBinderProvider());
    options.ModelBinderProviders.Add(new KeyValuePairModelBinderProvider());
    options.ModelBinderProviders.Add(new DictionaryModelBinderProvider());
    options.ModelBinderProviders.Add(new ArrayModelBinderProvider());
    options.ModelBinderProviders.Add(new CollectionModelBinderProvider());
    options.ModelBinderProviders.Add(new ComplexTypeModelBinderProvider());

    ...
}

以上便是.NET Core MVC中所有被框架支持的ModelBinderProvider。

以一個最典型的FormCollectionModelBinderProvider為例。它以Metadata.ModelType的類型作為判斷依據，如果是IFormCollection類型的話，則返回一個FormCollectionModelBinder對象。

public IModelBinder GetBinder(ModelBinderProviderContext context)
{
    ...

    var modelType = context.Metadata.ModelType;

    ...

    if (modelType == typeof(IFormCollection))
    {
        var loggerFactory = context.Services.GetRequiredService<ILoggerFactory>();
        return new FormCollectionModelBinder(loggerFactory);
    }

    return null;
}

在CreateBinderCoreUncached方法的循環體內部會依次嘗試ModelBinderProvider們是否能創建合適的ModelBinder，一旦能夠生成ModelBinder，則跳出當前循環，以這個對象作為返回值。

ValueProvider

有了ModelBinder，還需要有數據才能進行綁定。而為ModelBinder提供數據的是一些ValueProvider。

MvcCoreMvcOptionsSetup類的Configure方法裏，再往下找，可以看到ValueProvider們的蹤影。更確切地是與之對應的工廠類們。

public void Configure(MvcOptions options)
{
    ...

    // Set up ValueProviders
    options.ValueProviderFactories.Add(new FormValueProviderFactory());
    options.ValueProviderFactories.Add(new RouteValueProviderFactory());
    options.ValueProviderFactories.Add(new QueryStringValueProviderFactory());
    options.ValueProviderFactories.Add(new JQueryFormValueProviderFactory());

    ...
}

以FormValueProviderFactory為例，看一下其內部：

public Task CreateValueProviderAsync(ValueProviderFactoryContext context)
{
    ...

    var request = context.ActionContext.HttpContext.Request;
    if (request.HasFormContentType)
    {
        // Allocating a Task only when the body is form data.
        return AddValueProviderAsync(context);
    }

    return Task.CompletedTask;
}

private static async Task AddValueProviderAsync(ValueProviderFactoryContext context)
{
    var request = context.ActionContext.HttpContext.Request;
    var valueProvider = new FormValueProvider(
        BindingSource.Form,
        await request.ReadFormAsync(),
        CultureInfo.CurrentCulture);

    context.ValueProviders.Add(valueProvider);
}

通過CreateValueProviderAsync方法可以得到一個FormValueProvider對象。

而這些ValueProviderFactory所創建的ValueProvider又統一被CompositeValueProvider類的CreateAsync方法聚合成CompositeValueProvider這個集合對象的內部元素。

public static async Task<CompositeValueProvider> CreateAsync(
    ActionContext actionContext,
    IList<IValueProviderFactory> factories)
{
    var valueProviderFactoryContext = new ValueProviderFactoryContext(actionContext);

    for (var i = 0; i < factories.Count; i++)
    {
        var factory = factories[i];
        await factory.CreateValueProviderAsync(valueProviderFactoryContext);
    }

    return new CompositeValueProvider(valueProviderFactoryContext.ValueProviders);
}

再到ControllerBinderDelegateProvider類的CreateBinderDelegate方法中，找到valueProvider創建的起始點。

async Task Bind(ControllerContext controllerContext, object controller, Dictionary<string, object> arguments)
{
    var valueProvider = await CompositeValueProvider.CreateAsync(controllerContext);
    var parameters = actionDescriptor.Parameters;

    for (var i = 0; i < parameters.Count; i++)
    {
        var parameter = parameters[i];
        var bindingInfo = parameterBindingInfo[i];
        var modelMetadata = bindingInfo.ModelMetadata;

        if (!modelMetadata.IsBindingAllowed)
        {
            continue;
        }

        var result = await parameterBinder.BindModelAsync(
            controllerContext,
            bindingInfo.ModelBinder,
            valueProvider,
            parameter,
            modelMetadata,
            value: null);

        if (result.IsModelSet)
        {
            arguments[parameter.Name] = result.Model;
        }
    }

    ...
}

所得到的valueProvider在ParameterBinder類的BindModelAsync方法裏還要再作進一步的處理。先作為參數傳入創建DefaultModelBindingContext的方法：

var modelBindingContext = DefaultModelBindingContext.CreateBindingContext(
    actionContext,
    valueProvider,
    metadata,
    parameter.BindingInfo,
    parameter.Name);

再對ValueProvider作過濾處理：

return new DefaultModelBindingContext()
{
    ActionContext = actionContext,
    BinderModelName = binderModelName,
    BindingSource = bindingSource,
    PropertyFilter = propertyFilterProvider?.PropertyFilter,

    // Because this is the top-level context, FieldName and ModelName should be the same.
    FieldName = binderModelName ?? modelName,
    ModelName = binderModelName ?? modelName,

    IsTopLevelObject = true,
    ModelMetadata = metadata,
    ModelState = actionContext.ModelState,

    OriginalValueProvider = valueProvider,
    ValueProvider = FilterValueProvider(valueProvider, bindingSource),

    ValidationState = new ValidationStateDictionary(),
};

FilterValueProvider方法最終會調用CompositeValueProvider類的Filter方法，以得到所有合適的valueProvider。

public IValueProvider Filter(BindingSource bindingSource)
{
    ...

    var filteredValueProviders = new List<IValueProvider>();
    foreach (var valueProvider in this.OfType<IBindingSourceValueProvider>())
    {
        var result = valueProvider.Filter(bindingSource);
        if (result != null)
        {
            filteredValueProviders.Add(result);
        }
    }

    ...

    return new CompositeValueProvider(filteredValueProviders);
}

那麽當在ModelBinder的BindModelAsync方法裏需要獲取數據時，以FloatModelBinder為例：

public Task BindModelAsync(ModelBindingContext bindingContext)
{
    ...

    var modelName = bindingContext.ModelName;
    var valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);
    
    ...
}

會試圖從已過濾的ValueProvider中獲取值。這時還是利用了CompositeValueProvider類中的方法。

public virtual ValueProviderResult GetValue(string key)
{
    // Performance-sensitive
    // Caching the count is faster for IList<T>
    var itemCount = Items.Count;
    for (var i = 0; i < itemCount; i++)
    {
        var valueProvider = Items[i];
        var result = valueProvider.GetValue(key);
        if (result != ValueProviderResult.None)
        {
            return result;
        }
    }

    return ValueProviderResult.None;
}

這裏的邏輯是從valueProvider集合中逐一嘗試取值，有數據的則直接返回。

這也意味著數據綁定會以FormValueProvider到RouteValueProvider，再到QueryStringValueProvider，最後向JQueryFormValueProvider取值，這一流程執行，中間如果有任何一個能得到數據的話，則不再繼續訪問後面的ValueProvider。當然，前提是這些ValueProvider要不被先前的過濾處理排除在外。

若是還不明白這一順序關系的話，可以回想下從ValueProviderFactories的添加順序，再至ValueProvider集合生成時各個ValueProvider的順序，就比較容易了解其中道理了。

.NET Core開發日誌——Model Binding