CL Integration function that will take an Image/Varbinary value as input

I am trying to create a CLR Integration function (SQL Server 2008 R2) which will read a Image/Varbinary value and return a NVARCHAR(MAX).

    [Microsoft.SqlServer.Server.SqlFunction]
    public static SqlString ConvertByteArrToString(SqlBinary arrByte)

And in SQL Server -

    create function ReadAsString(@varData varbinary(max))
    returns nvarchar(max)
    as
    external name CLRFunctions.[CLRFunctions.FormatUtilities].ConvertByteArrToString

But, when I try to create above funtion - I get below error.

    Msg 6552, Level 16, State 3, Procedure ReadAsString, Line 1
    ALTER FUNCTION for "ReadAsString" failed because T-SQL and CLR types for parameter "@varData" do not match.

Can someone suggest what should be the correct CLR match for SQL=Server Image/Varbinary(MAX).