Most simple lazy implementation. (Contains a Linqad Main methods for verification)

SimpleLazy.cs

void Main()
{
	var lazy = new SimpleLazy<string>(() =>
	{
		"calculating".Dump();
		return "hello";
	});
	lazy.Value.Dump("1");
	lazy.Value.Dump("2");
	
	var lazy2 = new SimpleLazy<int>(null);
}

// Define other methods and classes here

public sealed class SimpleLazy<T>
{
	private readonly Func<T> calculateValue;
	private bool isCalculated = false;
	private T value;
	
	public SimpleLazy(Func<T> calculateValue)
	{
		this.calculateValue = calculateValue ?? throw new ArgumentNullException(nameof(calculateValue));
	}
	
	public T Value => this.isCalculated ? this.value : this.Calculate();
	
	private T Calculate()
	{
		this.value = this.calculateValue();
		this.isCalculated = true;
		return this.value;
	}
}