Generics in C using type of a variable as parameter

Question

I have a generic method   bool DoesEntityExist lt T gt  Guid guid  ITransaction transaction  where T   IGloballyIdentifiable    How do I use the method in the following way   Type t   entity GetType    DoesEntityExist lt t gt  entityGuid  transaction     I keep receiving the foollowing compile error      The type or namespace name  t  could   not be found  are you missing a using   directive or an assembly reference     DoesEntityExist lt MyType gt  entityGuid  transaction     works perfectly but I do not want to use an if directive to call the method with a separate type name every time

User · Answer

One way to get around this is to use implicit casting:

bool DoesEntityExist<T>(T entity, Guid guid, ITransaction transaction) where T : IGloballyIdentifiable;

calling it like so:

DoesEntityExist(entity, entityGuid, transaction);

Going a step further, you can turn it into an extension method (it will need to be declared in a static class):

static bool DoesEntityExist<T>(this T entity, Guid guid, ITransaction transaction) where T : IGloballyIdentifiable;

calling as so:

entity.DoesEntityExist(entityGuid, transaction);

User · Answer

I m not sure whether I understand your question correctly  but you can write your code in this way   bool DoesEntityExist lt T gt  T instance          You can call the method in following fashion   DoesEntityExist myTypeInstance         This way you don t need to explicitly write the type  the framework will overtake the type automatically from the instance

User · Answer

The point about generics is to give compile-time type safety - which means that types need to be known at compile-time   You can call generic methods with types only known at execution time  but you have to use reflection      For non-public methods  you ll need to specify binding flags too MethodInfo method   GetType   GetMethod  DoesEntityExist                                 MakeGenericMethod new Type     t     method Invoke this  new object     entityGuid  transaction       Ick   Can you make your calling method generic instead  and pass in your type parameter as the type argument  pushing the decision one level higher up the stack   If you could give us more information about what you re doing  that would help  Sometimes you may need to use reflection as above  but if you pick the right point to do it  you can make sure you only need to do it once  and let everything below that point use the type parameter in a normal way

User · Answer

You can t use it in the way you describe  The point about generic types  is that although you may not know them at  coding time   the compiler needs to be able to resolve them at compile time  Why  Because under the hood  the compiler will go away and create a new type  sometimes called a closed generic type  for each different usage of the  open  generic type   In other words  after compilation   DoesEntityExist lt int gt    is a different type to  DoesEntityExist lt string gt    This is how the compiler is able to enfore compile-time type safety   For the scenario you describe  you should pass the type as an argument that can be examined at run time   The other option  as mentioned in other answers  is that of using reflection to create the closed type from the open type  although this is probably recommended in anything other than extreme niche scenarios I d say

[c#] Generics in C#, using type of a variable as parameter

Examples related to c#

Examples related to .net

Examples related to generics

Examples related to types