用.NET语言写一个计算股票技术指标DMI的函数，第一个参数为datatable类型，日期列名为“日期”，开盘价列名为“开盘”,收盘价列名为“收盘”,最高价列名为“最高”,最低价列名为“最低”，返回值为datatable类型，返回值包含PDI,MDI,ADX,ADXR等列，double类型全部四舍五入保留3位小数。

好的，以下是计算DMI指标的函数代码：

public static DataTable CalculateDMI(DataTable dt)
{
    // 计算方向线DI和动态区间范围TR
    double[] DIp = new double[dt.Rows.Count];
    double[] DIn = new double[dt.Rows.Count];
    double[] TR = new double[dt.Rows.Count];
    for (int i = 1; i < dt.Rows.Count; i++)
    {
        double high = Convert.ToDouble(dt.Rows[i]["最高"]);
        double low = Convert.ToDouble(dt.Rows[i]["最低"]);
        double pre_close = Convert.ToDouble(dt.Rows[i - 1]["收盘"]);
        double pdm = Math.Max(high - pre_close, 0);
        double ndm = Math.Max(pre_close - low, 0);
        DIp[i] = pdm;
        DIn[i] = ndm;
        TR[i] = Math.Max(Math.Max(high - low, Math.Abs(high - pre_close)), Math.Abs(low - pre_close));
    }

    // 计算平均动向指标ADX和动向指标DI
    double[] DX = new double[dt.Rows.Count];
    double[] ADX = new double[dt.Rows.Count];
    double[] ADXR = new double[dt.Rows.Count];
    for (int i = 14; i < dt.Rows.Count; i++)
    {
        double sum_dip = 0, sum_din = 0, sum_tr = 0;
        for (int j = i - 13; j <= i; j++)
        {
            sum_dip += DIp[j];
            sum_din += DIn[j];
            sum_tr += TR[j];
        }
        double pdi = 100 * sum_dip / sum_tr;
        double ndi = 100 * sum_din / sum_tr;
        double dx = 100 * Math.Abs(pdi - ndi) / (pdi + ndi);
        DX[i] = dx;
        if (i == 14 + 14 - 1)
        {
            double sum_dx = 0;
            for (int j = i - 13; j <= i; j++)
            {
                sum_dx += DX[j];
            }
            ADX[i] = sum_dx / 14;
        }
        else if (i > 14 + 14 - 1)
        {
            ADX[i] = (ADX[i - 1] * 13 + DX[i]) / 14;
        }
        if (i >= 14 + 14 - 1 + 5)
        {
            ADXR[i] = (ADX[i] + ADX[i - 5]) / 2;
        }
    }

    // 构造返回结果
    DataTable result = new DataTable();
    result.Columns.Add("日期", typeof(DateTime));
    result.Columns.Add("PDI", typeof(double));
    result.Columns.Add("MDI", typeof(double));
    result.Columns.Add("ADX", typeof(double));
    result.Columns.Add("ADXR", typeof(double));
    for (int i = 14 + 14 - 1 + 5; i < dt.Rows.Count; i++)
    {
        DataRow row = result.NewRow();
        row["日期"] = dt.Rows[i]["日期"];
        row["PDI"] = Math.Round(100 * DIp[i] / TR[i], 3);
        row["MDI"] = Math.Round(100 * DIn[i] / TR[i], 3);
        row["ADX"] = Math.Round(ADX[i], 3);
        row["ADXR"] = Math.Round(ADXR[i], 3);
        result.Rows.Add(row);
    }
    return result;
}

需要注意的是，这个函数假定输入的数据表dt已经按照日期升序排列，而且至少包含14 + 14 - 1 + 5 = 32行数据（因为要计算ADX和ADXR需要至少32个数据点）。如果数据不足，函数会返回空的结果表。