private const double Precision = 1E-6; // 精度
private bool fSearchExpression(double[] ANumbers, string[] AExpressions,
    int ALevel, int ADest, List<string> AResults)
{
    bool Result = false;
    if ((ALevel <= 1) && (Math.Abs(ANumbers[0] - ADest) <= Precision))
    {
        AResults.Add(AExpressions[0]);
        return true;
    }
    for (int i = 0; i < ALevel; i++)
        for (int j = i + 1; j < ALevel; j++)
        {
            double A = ANumbers[i];
            double B = ANumbers[j];
            ANumbers[j] = ANumbers[ALevel - 1];
            string vExpA = AExpressions[i];
            string vExpB = AExpressions[j];
            AExpressions[j] = AExpressions[ALevel - 1];
            AExpressions[i] = '(' + vExpA + '+' + vExpB + ')';
            ANumbers[i] = A + B;
            if (fSearchExpression(ANumbers, AExpressions, 
                ALevel - 1, ADest, AResults)) Result = true;
            AExpressions[i] = '(' + vExpA + '-' + vExpB + ')';
            ANumbers[i] = A - B;
            if (fSearchExpression(ANumbers, AExpressions,
                ALevel - 1, ADest, AResults)) Result = true;
            AExpressions[i] = '(' + vExpB + '-' + vExpA + ')';
            ANumbers[i] = B - A;
            if (fSearchExpression(ANumbers, AExpressions,
                ALevel - 1, ADest, AResults)) Result = true;
            AExpressions[i] = '(' + vExpA + '*' + vExpB + ')';
            ANumbers[i] = A * B;
            if (fSearchExpression(ANumbers, AExpressions,
                ALevel - 1, ADest, AResults)) Result = true;
            if (B != 0)
            {
                AExpressions[i] = '(' + vExpA + '/' + vExpB + ')';
                ANumbers[i] = A / B;
                if (fSearchExpression(ANumbers, AExpressions,
                    ALevel - 1, ADest, AResults)) Result = true;
            }
            if (A != 0)
            {
                AExpressions[i] = '(' + vExpB + '/' + vExpA + ')';
                ANumbers[i] = B / A;
                if (fSearchExpression(ANumbers, AExpressions,
                    ALevel - 1, ADest, AResults)) Result = true;
            }
            ANumbers[i] = A;
            ANumbers[j] = B;
            AExpressions[i] = vExpA;
            AExpressions[j] = vExpB;
        }
    return Result;
}

private bool SearchExpression(List<string> AResults, int ADest, params int[] ANumbers)
{
    double[] vNumbers = new double[ANumbers.Length];
    string[] vExpressions = new string[ANumbers.Length];
    for (int i = 0; i < ANumbers.Length; i++)
    {
        vNumbers[i] = ANumbers[i];
        vExpressions[i] = ANumbers[i].ToString();
    }
    return fSearchExpression(vNumbers, vExpressions, ANumbers.Length, ADest, AResults);
}

private void button1_Click(object sender, EventArgs e)
{
    List<string> vExpressions = new List<string>();
    SearchExpression(vExpressions, 24, 4, 4, 7, 7);
    foreach (string vExpression in vExpressions)
        textBox1.AppendText(vExpression + "\r\n");
}

------解决方案--------------------
探讨

List <string>
这个代表什么意思。。初学者