Foreach loop in ssis script component

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C#

SSIS

Hi, I have the following codes in script component in ssis data flow. "PartNo" will be passed in as:
ABC
DEF
GHI

How can I concatenate them into one string like ABC,DEF,GHI?
After I have written my script as below, it prompts me an error message
Below are my script:

public override void Input0_ProcessInputRow(Input0Buffer Row)
{
    string PartNo = Row.PartNo;
    foreach (string partNumber in Row.PartNo)
    {
        Output0Buffer.AddRow();
        Output0Buffer.PartNoAll = PartNo;
    }
}

If I remove the foreach loop, there is no error but the output is not as I desired.
Any help would be appreciated. Thank you.