Telling the difference between List<string> and List<T>?

Question

I have an application that I just want to check the Cont property of List<T> but the object sent in is an Object. So the question is two fold.

1) How do I tell that the object is a List<T>? I can specifically test for List<string> (like if(o is List<string>). But I don't know how to test if it is a generic list?

2) Once I know that the object is a List<T> I want to find out how many there are so I need to cast it to List<T> in order to call Count. Again I am not sure how to do this.

 

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Kevin Burton

Answer 1

To define the List as a list of objects [ I do this all the time]:

Define a class with your methods and properties for the class [ Data fields ]

then in the program that is using the class:

List<Rcd> rcds = new List<Rcd>(); // this defines the list to store the Rcd class structure.

// then to use the list...

while(sql.Reader.Read())

{

   Rcd r = new Rcd(sql.Reader); // the constructor takes the Sql data Reader object and populates the class

   rcds.Add(r); // Adds the Rcd class object to the list of rcds.

}

 

.....

Second Question: answer is simple....use the rcds.Length proprty...This will determine the number on entries in rcds array.

 

Fred

Answer 2

A better option is to just test for ICollection.

 

Since List<T> implements ICollection, any List<T> (plus many other classes) will return true if they're an ICollection, which will allow you to get a count directly.

 

Just do:

 

int count = 0;

ICollection collection = o as ICollection;

if (collection != null)

    count = collection.Count;

 

 

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Reed Copsey, Jr. - http://reedcopsey.com
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Answer 3

Fred.Simons.ITG,

I think you misunderstood my question. I have an object that is presented to me as a type object I need to verify that the object is a List<T> and that the number of items in the list is greater than zero. I think Red Copsey, Jr posted a solution that should work. Thank you.

 

---

Kevin Burton

Answer 4

You can check if the object is a List<T> with 

Type objType = obj.GetType();
 bool isList = objType.IsGenericType == true &&
   objType.GetGenericTypeDefinition() != typeof(List<>);

Get the type of the list with

Type itemType = propertyType.GetGenericArguments()[0];

Test for string with

bool itemIsString = itemType == typeof(string);

For Count you cast to IList or ICollection as suggested.

Hope this helps.