接着上文Asp.net web Api源码分析-Filter 我们提到filter的获取和调用，后面通过HttpActionBinding actionBinding = actionDescriptor.ActionBinding;来获取HttpActionBinding实例，然后调用 HttpActionBinding的ExecuteBindingAsync方法来绑定Action参数。HttpActionDescriptor中 定义了ActionBinding属性，默认的实现代码：

ServicesContainer controllerServices = _controllerDescriptor.Configuration.Services;
 IActionValueBinder actionValueBinder = controllerServices.GetActionValueBinder();
 HttpActionBinding actionBinding = actionValueBinder.GetBinding(this);

这里的actionValueBinder默认就是一个DefaultActionValueBinder实例，然后调用它的GetBinding方法。然我们看看DefaultActionValueBinder的GetBinding方法：

public virtual HttpActionBinding GetBinding(HttpActionDescriptor actionDescriptor)
        {
            if (actionDescriptor == null)
            {
                throw Error.ArgumentNull("actionDescriptor");
            }

HttpParameterDescriptor[] parameters = actionDescriptor.GetParameters().ToArray();
            HttpParameterBinding[] binders = Array.ConvertAll(parameters, GetParameterBinding);

HttpActionBinding actionBinding = new HttpActionBinding(actionDescriptor, binders);

EnsureOneBodyParameter(actionBinding);

return actionBinding;
        }

这里首先获取当前HttpActionDescriptor的参数集合ReflectedHttpParameterDescriptor[],然后依次调用GetParameterBinding方法把当前HttpActionDescriptor转化为HttpParameterBinding，GetParameterBinding方法如下：

  protected virtual HttpParameterBinding GetParameterBinding(HttpParameterDescriptor parameter){// Attribute has the highest precedence// Presence of a model binder attribute overrides.ParameterBindingAttribute attr = parameter.ParameterBinderAttribute;if (attr != null){return attr.GetBinding(parameter);}// No attribute, so lookup in global map.ParameterBindingRulesCollection pb = parameter.Configuration.ParameterBindingRules;if (pb != null){HttpParameterBinding binding = pb.LookupBinding(parameter);if (binding != null){return binding;}}// Not explicitly specified in global map or attribute.// Use a default policy to determine it. These are catch-all policies. Type type = parameter.ParameterType;if (TypeHelper.IsSimpleUnderlyingType(type) || TypeHelper.HasStringConverter(type)){// For simple types, the default is to look in URI. Exactly as if the parameter had a [FromUri] attribute.return parameter.BindWithAttribute(new FromUriAttribute());}// Fallback. Must be a complex type. Default is to look in body. Exactly as if this type had a [FromBody] attribute.attr = new FromBodyAttribute();return attr.GetBinding(parameter);}

这里我们首先调用 ParameterBindingAttribute attr = parameter.ParameterBinderAttribute;来获取ParameterBindingAttribute实例，然后调用它的 GetBinding方法来获取HttpParameterBinding，我们还是来看看ParameterBinderAttribute是如何定义 了吧：

在HttpParameterDescriptor中的ParameterBinderAttribute属性定义如下：

public virtual ParameterBindingAttribute ParameterBinderAttribute
        {
            get
            {
                if (_parameterBindingAttribute == null)
                {
                    if (!_searchedModelBinderAttribute)
                    {
                        _searchedModelBinderAttribute = true;
                        _parameterBindingAttribute = FindParameterBindingAttribute();
                    }
                }

return _parameterBindingAttribute;
            }

set { _parameterBindingAttribute = value; }
        }

FindParameterBindingAttribute其实就是查找参数或则参数类型的ParameterBindingAttribute特性，如果这里取到的ParameterBindingAttribute属性多余一个则最后要抛出异常，具体这里就不说了。

回到DefaultActionValueBinder中GetParameterBinding方法，找不到 ParameterBindingAttribute对象，我们就 parameter.Configuration.ParameterBindingRules来获取 ParameterBindingRulesCollection实例，其中 ParameterBindingRules=DefaultActionValueBinder.GetDefaultParameterBinders()， 其中GetDefaultParameterBinders方法实现如下：

internal static ParameterBindingRulesCollection GetDefaultParameterBinders()
        {
            ParameterBindingRulesCollection pb = new ParameterBindingRulesCollection();

pb.Add(typeof(CancellationToken), parameter => new CancellationTokenParameterBinding(parameter));
            pb.Add(typeof(HttpRequestMessage), parameter => new HttpRequestParameterBinding(parameter));

// Warning binder for HttpContent.
            pb.Add(parameter => typeof(HttpContent).IsAssignableFrom(parameter.ParameterType) ?
                                    parameter.BindAsError(Error.Format(SRResources.ParameterBindingIllegalType, parameter.ParameterType.Name, parameter.ParameterName))
                                    : null);

return pb;
        }

这里的ParameterBindingRulesCollection实例有3个成员，然后调用 ParameterBindingRulesCollection的LookupBinding方法来获取HttpParameterBinding实 例，其中LookupBinding方法如下：

public class ParameterBindingRulesCollection : Collection<Func<HttpParameterDescriptor, HttpParameterBinding>>
{
     private static Func<HttpParameterDescriptor, HttpParameterBinding> TypeCheck(Type type, Func<HttpParameterDescriptor, HttpParameterBinding> func)
    {
        return (param => (param.ParameterType == type) ? func(param) : null);
    }
    public void Add(Type typeMatch, Func<HttpParameterDescriptor, HttpParameterBinding> funcInner)
    {
        Add(TypeCheck(typeMatch, funcInner));
    }
    public HttpParameterBinding LookupBinding(HttpParameterDescriptor parameter)
    {
        foreach (Func<HttpParameterDescriptor, HttpParameterBinding> func in this)
        {
            HttpParameterBinding binding = func(parameter);
            if (binding != null)
            {
                 return binding;
            }
        }
        return null;
    }
}

所以这里默认的ParameterBindingRulesCollection3个成员是不会返回HttpParameterBinding实例。

如果参数类型是一个简单类型，并且该类型可以转化为string类型，然后调用  return parameter.BindWithAttribute(new FromUriAttribute());返回HttpParameterBinding，BindWithAttribute方法其实就是调用ParameterBindingAttribute的GetBinding方法，这里默认FromUriAttribute的GetBinding方法，这里FromUriAttribute的继承数如下：FromUriAttribute-》ModelBinderAttribute-》ParameterBindingAttribute。这里的

如果绑定的参数数据类型比较特殊，那么这里我们就调用FromBodyAttribute的GetBinding方法来获取HttpParameterBinding实例，这里的FromBodyAttribute继承树如下：

FromBodyAttribute-》ParameterBindingAttribute

在这里我们总结一下这里找HttpParameterBinding的顺序，（1）parameter.ParameterBinderAttribute实际就是找参数或参数类型的ParameterBindingAttribute属性，（2）

parameter.Configuration.ParameterBindingRules 从全局的ParameterBindingRules中找HttpParameterBinding，（3）如果参数类型是一个简单类型且可以转化为 string那么我们调用parameter.BindWithAttribute(new FromUriAttribute())，（4）最后我们调用FromBodyAttribute的GetBinding方法来获取 HttpParameterBinding实例。

现在我们回到DefaultActionValueBinder的GetBinding方法中来，现在我们已经获取到HttpParameterBinding集合，接下里创建一个HttpActionBinding实例，最后调用EnsureOneBodyParameter来检查HttpActionBinding的ParameterBindings集合是否有2个都需要读取form表单，如果是则抛出异常。到这里HttpActionDescriptor的ActionBinding的创建也就很清楚了。

回到ApiController的ExecuteAsync方法中来，这里继续调用HttpActionBinding的ExecuteBindingAsync方法，这里的ExecuteBindingAsync方法实现如下：

public virtual Task ExecuteBindingAsync(HttpActionContext actionContext, CancellationToken cancellationToken)
        {
            if (_parameterBindings.Length == 0)
            {
                return TaskHelpers.Completed();
            }

// First, make sure the actionBinding is valid before trying to execute it. This keeps us in a known state in case of errors.
            foreach (HttpParameterBinding parameterBinder in ParameterBindings)
            {
                if (!parameterBinder.IsValid)
                {
                    // Throwing an exception because the webService developer's action signature is bad.
                    // This exception will be caught and converted into a 500 by the dispatcher
                    return TaskHelpers.FromError(new InvalidOperationException(parameterBinder.ErrorMessage));
                }
            }

if (_metadataProvider == null)
            {
                HttpConfiguration config = actionContext.ControllerContext.Configuration;
                _metadataProvider = config.Services.GetModelMetadataProvider();
            }

// Execute all the binders.
            IEnumerable<Task> tasks = from parameterBinder in ParameterBindings select parameterBinder.ExecuteBindingAsync(_metadataProvider, actionContext, cancellationToken);
            return TaskHelpers.Iterate(tasks, cancellationToken, disposeEnumerator: false);
        }

这个方法逻辑很简单，如果ParameterBindings没有成员则直接返回，如果有则需要依次验证他们的IsValid，然后再依次调用他们的ExecuteBindingAsync方法，ExecuteBindingAsync方法的具体执行我们这里就不多说了。这里我们看看metadataProvider 是个什么东东吧，     SetSingle<ModelMetadataProvider>(new DataAnnotationsModelMetadataProvider());这里我们就知道metadataProvider 其实是一个DataAnnotationsModelMetadataProvider实例，其构造函数也没什么特别的。这里返回的Task的具体实现我就不多说了，里面用到了一个TaskCompletionSource类，具体的使用我这里一而不说了。