Page 190 - CSharp/C#

["foo"] = 34,
["bar"] = 42
};

Any object that has an indexed getter or setter can be used with this syntax:

class Program
{
public class MyClassWithIndexer
{
public int this[string index]
{
set
{
Console.WriteLine($"Index: {index}, value: {value}");
}
}
}

public static void Main()
{
var x = new MyClassWithIndexer()
{
["foo"] = 34,
["bar"] = 42
};

Console.ReadKey();
}
}

Output:

Index: foo, value: 34
Index: bar, value: 42

View Demo

If the class has multiple indexers it is possible to assign them all in a single group of statements:

class Program
{
public class MyClassWithIndexer
{
public int this[string index]
{
set
{
Console.WriteLine($"Index: {index}, value: {value}");
}
}
public string this[int index]
{
set
{
Console.WriteLine($"Index: {index}, value: {value}");
}

https://riptutorial.com/ 136