I have an interface as IRedisHelper<T>. I want cast T to my Specific entity class. but it gives the error :

'T' is a type, which is not valid in the given context

 public interface IRedisHelper<T> where T : Entity
        {
            string GetCacheKey();
        }

   

         public class RedisHelper<T> : IRedisHelper<T> where T : Entity
            {
                public string GetCacheKey()
                {
                    try
                    {
                        string cacheKey = string.Empty;
                        var entity = typeof(T);
                        switch (true)
                        {
                            case var _ when entity == typeof(AppSlider):
                       cacheKey = 
                         Constants.GetAppSlidersCacheKey   (T as AppSlider).Id;
                                break;
                                //...
                        }
                        return cacheKey;
                    }
                    catch (Exception ex)
                    {
                        throw ex;
                    }
                }
            }

Here is Entity Models:

 public abstract class Entity
  {
     public long Id { get; set; }
  }

  public class AppSlider : Entity
   {
       public string Title { get; set; }
   }

How Could I cast T to my class 'AppSlider' and get AppSlider.Id?

CodePudding user response：

Your RedisHelper class is going to need some sort of field or property that stores whatever T is that has been passed. That, or your GetCacheKey() method needs a parameter with T.

This is because your trying to access object data, but have no reference to a specific object (only Type info).

Consider making a constructor that accepts a parameter of T, like this:

private T entityInstance;

public RedisHelper(T entityData)
{
  entityInstance = entityData;
}

Then you can use the instance-specific item here:

switch (entityInsance)
{
  case AppSlider appSliderInstance:
    cacheKey = Constants.GetAppSlidersCacheKey   appSliderInstance.Id.ToString();
  break;
  //...
}

CodePudding user response：

The error is clear, as operator is for casting objects which means you can cast an instance (not Type) to an object (or its class), so you need an instance of T.

I think the best way is to pass the entity to GetCacheKey method like this:

public interface IRedisHelper<T> where T : Entity
{
    string GetCacheKey(T tEntity);
}



public class RedisHelper<T> : IRedisHelper<T> where T : Entity
{
    public string GetCacheKey(T tEntity)
    {
        try
        {
            string cacheKey = string.Empty;
            var entity = typeof(T);
            switch (true)
            {
                case var _ when entity == typeof(AppSlider):
                    cacheKey =
                        Constants.GetAppSlidersCacheKey   tEntity.Id;
                    break;
                //...
            }
            return cacheKey;
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }
}