Error in system.security.cryptography.rsa does not contain definition for encrypt

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C#

cryptography

RSA

I was trying out the RSA encryption code, referring to some of the codes online, and I tried this

var encryptedData = RSA.Encrypt(testDatabyte, publicKey)

but error came out mentioning that

System.Security.Cryptography.RSA does not contain definition for Encrypt

I am using .NET framework 4.5.1. Can i know how should I proceed?

What I have tried:

-referred to msdn microsoft and tried to add assemblies like
System.Security.Cryptography.AsymmetricAlgorithm
System.Security.Cryptography.RSA
System.Security.Cryptography.RSACryptoServiceProvider
but all of them is a type not a namespace
-have check the .Net framework version in msdn, and it did mention it is supported