How to Call a Generic Method Using a Type Variable in C#?

C# .net Generics Reflection

Introduction

When working with generic methods in C#, there may be instances where the type parameter is not known at compile time but is obtained dynamically at runtime. In such cases, it can be challenging to find the most concise way to invoke the generic method. This article aims to provide a solution to this problem and guide you through the process of calling a generic method using a Type variable in C#.

Problem Description

Consider the following sample code: 


                public class Sample
                {
                    public void Example(string typeName)
                    {
                        Type myType = FindType(typeName);

                        // What goes here to call GenericMethod()?
                        GenericMethod<myType>(); // This doesn't work

                        // What changes to call StaticMethod()?
                        Sample.StaticMethod<myType>(); // This also doesn't work
                    }

                    public void GenericMethod<T>()
                    {
                        // ...
                    }

                    public static void StaticMethod<T>()
                    {
                        //...
                    }
                }
Inside the Example() method, we need to call the GenericMethod<T>() using the Type stored in the myType variable. However, directly substituting the type variable as seen in the code (GenericMethod<myType>()) is not valid and throws a compile-time error.

The "typeof" Operator

In order to call a generic method using a Type variable, we need to make use of the "typeof" operator. The "typeof" operator returns a Type instance representing the type passed as an operand. This allows us to obtain the required Type object dynamically at runtime.

Here's how we can modify the Example() method to call the GenericMethod<T>() using the myType variable: 


                public void Example(string typeName)
                {
                    Type myType = FindType(typeName);
                    
                    MethodInfo genericMethod = typeof(Sample).GetMethod("GenericMethod");
                    MethodInfo constructedMethod = genericMethod.MakeGenericMethod(myType);
                    constructedMethod.Invoke(this, null);
                }
In the modified code snippet, we first obtain the MethodInfo object of the GenericMethod<T>() using the GetMethod() method of the Type class. We pass the name of the method as a string parameter to this method. Next, we use the MakeGenericMethod() method to create a constructed method based on the myType. Finally, we invoke the constructed method using the Invoke() method and pass null as the arguments since the method doesn't require any parameters.

Calling a Static Generic Method

If the generic method is static, we need to modify the calling code accordingly. Here's how we can call the StaticMethod<T>() using a Type variable: 


                public void Example(string typeName)
                {
                    Type myType = FindType(typeName);
                    
                    MethodInfo genericMethod = typeof(Sample).GetMethod("StaticMethod");
                    MethodInfo constructedMethod = genericMethod.MakeGenericMethod(myType);
                    constructedMethod.Invoke(null, null);
                }
In the above code, we follow the same steps as before to obtain the MethodInfo object and create a constructed method using the MakeGenericMethod() method. However, this time we invoke the constructed method by passing null as the target object since the method is static.

Example Usage

Let's consider an example to demonstrate the usage of calling a generic method using a Type variable. 


            public class Sample
            {
                public T GetDefaultValue<T>()
                {
                    return default(T);
                }
            }
In the above code, we define a generic method GetDefaultValue() that returns the default value of type T. We can now call this method using a Type variable as shown below: 

            public static void Main()
            {
                Sample sample = new Sample();
                Type type = typeof(int);

                MethodInfo genericMethod = typeof(Sample).GetMethod("GetDefaultValue");
                MethodInfo constructedMethod = genericMethod.MakeGenericMethod(type);
                var result = constructedMethod.Invoke(sample, null);

                Console.WriteLine("Default value of type int: " + result);
            }
In this example, we create an instance of the Sample class and obtain the Type object for the int type. We then use reflection to call the GetDefaultValue() method by substituting the Type variable. The result is the default value of int, which in this case will be 0.

Using the same pattern, you can call other generic methods with different type parameters dynamically.

Conclusion

In this article, we explored how to call a generic method using a Type variable in C#. By utilizing the "typeof" operator, we can obtain the required Type object dynamically at runtime. With the help of the MethodInfo class and its methods like GetMethod() and MakeGenericMethod(), we can create a constructed method and invoke it, even when the type parameter is not known at compile time. This allows for greater flexibility and dynamism in your code.

Related Questions