foreach の変数は読み取り専用なんですか？

#region CodeItem Class

//**************************************************************************

// CodeItem Class

//**************************************************************************

public class CodeItem

{

    #region Constructor

    //**************************************************************************

    // Constructor

    //**************************************************************************

    private List<string>    convertCodes;

    private    string            convertCode;



    public CodeItem() : base() {}



    public CodeItem(string sourceCode) {

        this.Source = sourceCode;

    }



    #endregion



    #region Source Property

    //**************************************************************************

    // Source Property

    //**************************************************************************

    public string Source {

        get {

            return this._Source;

        }

        private set {

            this._Source = value;

            this.ConvertSet(value);

        }

    }



    private string _Source;



    #endregion

    

    #region ConvertSet

    //**************************************************************************

    // ConvertSet

    //**************************************************************************

    private void ConvertSet(string code) {

        string s = code;



        string[] a = s.Split('\\\\'');

        s = a[0];    // Comment Delete



        s = s.Replace('\\\\t', ' '); // Tab to Space



        s = s.Trim();

        s = s.Trim(':');

        s = s.Trim();



        // " で囲まれた文字列（定数）は対象外にするため、偶数要素数のみ処理する

        string[] a = s.Split('\\\\"'); 

        for (int i = 0; i < a.Length; i = i + 2) {

            a[i] = a[i].ToUpper(); // 英字大文字にする。



        }

        this.convertCode = String.Join("\\\\"", a);

    }

    #endregion

    :

    :

List<string> programCode;

public void CodeConvert() {
    foreach(string s in this.programCode) {
        s = "aaa";
    }
}

List<string> programCode;

public void CodeConvert() {
    foreach(string s in this.programCode) {
        s = new String("aaa");
    }
}

public void CodeConvert() {
    foreach(string s in this.programCode) {
        this.programCode.Add("aaa");
    }
}