What is the C# version of VB.NET's InputBox?

Add a reference to Microsoft.VisualBasic, InputBox is in the Microsoft.VisualBasic.Interaction namespace:

using Microsoft.VisualBasic;
string input = Interaction.InputBox("Prompt", "Title", "Default", x_coordinate, y_coordinate);

Only the first argument for prompt is mandatory

You mean InputBox? Just look in the Microsoft.VisualBasic namespace.

C# and VB.Net share a common library. If one language can use it, so can the other.