Error: Cannot convert null to type parameter ‘T’

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Problem

You’re trying to return null from a generic method and you’re getting the following compiler error:

Cannot convert null to type parameter ‘T’ because it could be a non-nullable value type. Consider using ‘default(T)’ instead

You can’t return null because the compiler doesn’t know if T is nullable.

Solution

There are a few options for solving this depending your scenario.

Let’s say your generic method looks like the method below, and then let’s look at the options for solving this error.

public T Load<T>()
{
	return null;
}
Code language: C# (cs)

Option 1 – return default(T)

Return default(T) instead of null.

public T Load<T>()
{
	return default(T);
}
Code language: C# (cs)

This is the option suggested by the compiler error itself. Use this if you are allowing any type to be used.

If T is nullable, it’ll return null. Otherwise it’ll return the default for the specified type. For example, if you call this with Load<int>() it’ll return 0.

Option 2 – constrain T to a nullable type

Return null and constrain T to class.

public T Load<T>() where T : class
{
	return null;
}
Code language: C# (cs)

Use this option if you know you’re going to using this generic method with reference types – such as classes/interfaces.

For example, let’s say you’re loading multiple interfaces, like this:

var dataReceiverPlugin = typeLoader.Load<IDataReceiverPlugin>();
var dataSenderPlugin = typeLoader.Load<IDataSenderPlugin>();

public interface IDataReceiverPlugin
{

}
public interface IDataSenderPlugin
{

}
Code language: C# (cs)

Option 3 – constrain T to a specific class

Return null and constrain T to a specific class. 

public T Load<T>() where T : PluginBase
{
	return null;
}
Code language: C# (cs)

For example, you’d use this if you had an abstract base class, like PluginBase, and wanted to call your method on subclasses of PluginBase:

var dataReceiverPlugin = typeLoader.Load<DataLoadingPlugin>();

public class DataLoadingPlugin : PluginBase { }

public abstract class PluginBase
{ }
Code language: C# (cs)

Note: Constraining to an interface doesn’t allow you to return null – you’ll still have to specify the ‘class’ constraint.

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