Add Type不支持字典初始化语法?

添加类型:c:\用户….cs(7):无效的表达式项“[”

c: \Users…..cs(6):返回新字典{

c: \Users…..cs(7):>>[“aa”]=“123”,

c: \Users…..cs(8):[“bb”]=“235”

$Source = @"
using System.Collections.Generic;
public class BasicTest
{
  public static Dictionary<string, string> Add(int a, int b)
    {
        return new Dictionary<string, string>{ 
            ["aa"] = "123",
            ["bb"] = "235"
        };
    }
  public int Multiply(int a, int b)
    {
    return (a * b);
    }
}
"@

Add-Type -TypeDefinition $Source