c# - Cannot implicitly convert type 'void' to 'System.Collections.Generic.Dictionary<string,bool>

Question

This code works fine

Dictionary<string, bool> test = new Dictionary<string, bool>();
        test.Add("test string", true);

The following code throws this error : Cannot implicitly convert type 'void' to 'System.Collections.Generic.Dictionary

Dictionary<string, bool> test = new Dictionary<string, bool>().Add("test string", true);

why? what is the difference?

Answer 1

The return type of .Add is void

If you are chaining calls, the last expression becomes the return value of the whole statement.

The return value of new Dictionary<K, V>() is Dictionary<K, V>, then you call .Add on it and .Add returns nothing (void)

You can use object initialiser syntax to do this inline:

Dictionary<string, bool> test = new Dictionary<string, bool> 
{ 
    { "test string", true } 
};

Edit: more info, a lot of fluent syntax style frameworks will return the object you called the method on to allow you to chain:

e.g.

public class SomeFluentThing 
{
   public SomeFluentThing DoSomething()
   {
       // Do stuff
       return this;
   }

   public SomeFluentThing DoSomethingElse()
   {
       // Do stuff
       return this;
   }

}

So you can chain naturally:

SomeFluentThingVariable.DoSomething().DoSomethingElse();