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How to Resolve Error "CS0305: Using the generic type 'type' requires 'number' type arguments" in C#

The Compiler Error CS0305 is a Generics Arity error. The message reads: "Using the generic type 'TypeName' requires 'X' type arguments".

In C#, Generics allow you to define classes, interfaces, and methods with placeholders for data types (Type Parameters). These are denoted by angle brackets, e.g., <T> or <TKey, TValue>. When you use a generic type, you must provide the exact number of concrete types that the definition expects. If the class expects two types (like a Key and a Value) and you provide only one (or none), the compiler raises CS0305.

This guide explains how to match your usage of generics with their definitions.

Understanding Type Arguments​

When a class is defined as public class Box<T>, it has an "Arity" of 1. It requires one type argument. When a class is defined as public class Map<TKey, TValue>, it has an "Arity" of 2. It requires two type arguments.

The compiler strictly enforces this count. You cannot instantiate Map without telling the compiler what TKey and TValue are.

Scenario 1: Missing Type Arguments entirely​

This is common when developers accustomed to older versions of C# (or Java raw types) forget to specify the type for a generic collection like List<T>.

Example of error​

Attempting to use List without the angle brackets <>.

using System.Collections.Generic;

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0305: Using the generic type 'List<T>' requires 1 type arguments.
        // The compiler found 'System.Collections.Generic.List<T>', 
        // but you treated it like a non-generic class.
        List names = new List(); 
    }
}

Solution: Specify the Type​

Add angle brackets containing the type of data the list will hold.

using System.Collections.Generic;

public class Program
{
    static void Main()
    {
        // ✅ Correct: We specify that this list holds 'strings'.
        List<string> names = new List<string>();
    }
}
note

If you actually intended to use the old, non-generic ArrayList, you must use System.Collections.ArrayList instead of List.

Scenario 2: Incorrect Number of Arguments​

Some generic types require multiple arguments. Dictionary<TKey, TValue> is the classic example; it requires a type for the key and a type for the value.

Example of error​

Providing only one type when two are required.

using System.Collections.Generic;

public class Cache
{
    // ⛔️ Error CS0305: Using the generic type 'Dictionary<TKey, TValue>' 
    // requires 2 type arguments.
    // We only provided 'string'.
    public Dictionary<string> DataCache { get; set; }
}

Solution: Fill All Placeholders​

Check the definition of the class (hover over it in your IDE) to see how many types it needs.

using System.Collections.Generic;

public class Cache
{
    // ✅ Correct: Key is string, Value is object.
    public Dictionary<string, object> DataCache { get; set; }
}

Scenario 3: Naming Conflicts (Non-Generic vs Generic)​

It is legal in C# to have a non-generic class Data and a generic class Data<T> in the same namespace. However, if you only define Data<T>, but try to use it as Data, you get CS0305.

Example of error​

You defined a generic class but tried to use it as a standard class.

// Definition
public class Repository<T> 
{ 
    public void Add(T item) { }
}

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0305: 'Repository<T>' requires 1 type argument.
        // You cannot use 'Repository' without the <T>.
        Repository repo = new Repository();
    }
}

Solution​

Decide if you meant to use the generic version or if you forgot to define a non-generic version.

public class Program
{
    static void Main()
    {
        // ✅ Correct: Using the generic version with 'User'
        Repository<User> repo = new Repository<User>();
    }
}

public class User { }
tip

Arity Overloading: If you define public class Repository { } (non-generic) alongside Repository<T>, the error will disappear because new Repository() will resolve to the non-generic class.

Conclusion​

CS0305 is a simple counting error.

  1. Count the Placeholders: Look at the class definition. Does it have <T>, <T1, T2>, or <T1, T2, T3>?
  2. Match the Usage: Ensure your code provides exactly that many types inside the angle brackets <...>.
  3. Check for Missing <>: If you provided no brackets at all, you likely forgot that the class is generic.

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