C#读Serialport COM1 异常

异常提示：Length = “spCom.BaseStream.Length”引发了“System.NotSupportedException”类型的异常 base {System.SystemException} = {"流不支持查找。"}
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if (spCom.IsOpen && spCom.BytesToRead > 0)
open 了但是 BytesToRead 就显示0 断点查找就报上面的异常