.Net namespace

object:  System.Object

string:  System.String

bool:    System.Boolean

byte:    System.Byte

sbyte:   System.SByte

short:   System.Int16

ushort:  System.UInt16

int:     System.Int32

uint:    System.UInt32

long:    System.Int64

ulong:   System.UInt64

float:   System.Single

double:  System.Double

decimal: System.Decimal

char:    System.Char

https://stackoverflow.com/questions/7074/what-is-the-difference-between-string-and-string-in-c

.Net namespace

https://docs.microsoft.com/ko-kr/dotnet/api/index?view=netframework-4.7.2

System namespace

https://docs.microsoft.com/ko-kr/dotnet/api/system?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DKO-KR%26k%3Dk(System);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.6.1);k(DevLang-csharp)%26rd%3Dtrue&view=netframework-4.7.2