Here's a summary of the different parameter modifiers and how the behavior changes for each, when using them with value-typed and reference-typed variables.

• No modifier - value types
  • Copy of value passed to method
  • Method can read value
• No modifier - reference types
  • Reference to object passed to method
  • Method can read/write object contents
• ref modifier - value types
  • Reference to variable passed to method
  • Method can read/write variable
• ref modifier - reference types
  • Reference to object reference passed to method
  • Method can read/write object contents
  • Method can change reference to point to new object
• out modifier - value types
  • Reference to variable passed to method
  • Method must write new value to variable
• out modifier - reference types
  • Reference to object reference passed to method
  • Method must change reference to point to new object

原文地址：#273 - Parameter Modifier Summary