// 在控制器方法中
public ActionResult SomeAction()
{
    var metadataProvider = new DataAnnotationsModelMetadataProvider();
    var model = new YourModelClass(); // 你的模型实例
    
    // 获取包含实例的元数据
    ModelMetadata metadata = metadataProvider.GetMetadataForType(() => model, typeof(YourModelClass));
    
    // 使用metadata...
    return View(model);
}

public class CustomModelMetadataProvider : DataAnnotationsModelMetadataProvider
{
    protected override ModelMetadata CreateMetadata(
        IEnumerable<Attribute> attributes,
        Type containerType,
        Func<object> modelAccessor,
        Type modelType,
        string propertyName)
    {
        // 调用基类方法获取基本元数据
        var metadata = base.CreateMetadata(attributes, containerType, modelAccessor, modelType, propertyName);
        
        // 可以在这里访问模型实例
        if (modelAccessor != null)
        {
            var modelInstance = modelAccessor();
            // 基于实例的自定义逻辑...
        }
        
        return metadata;
    }
}

protected void Application_Start()
{
    ModelMetadataProviders.Current = new CustomModelMetadataProvider();
    // 其他初始化代码...
}

// 错误方式 - 不会包含实例值
var metadata = provider.GetMetadataForType(null, typeof(YourModelClass));

// 正确方式
var metadata = provider.GetMetadataForType(() => modelInstance, typeof(YourModelClass));

protected override ModelMetadata CreateMetadata(...)
{
    var metadata = base.CreateMetadata(...);
    
    var myAttribute = attributes.OfType<MyCustomAttribute>().FirstOrDefault();
    if (myAttribute != null)
    {
        // 应用自定义逻辑
    }
    
    return metadata;
}

private readonly ConcurrentDictionary<Type, ModelMetadata> _metadataCache = new();

public ModelMetadata GetMetadataForType(Func<object> modelAccessor, Type modelType)
{
    return _metadataCache.GetOrAdd(modelType, t => 
        base.GetMetadataForType(modelAccessor, t));
}