c# - 如何更改数据表中数据列的数据类型？

Question

我有：

DataTable Table = new DataTable;
SqlConnection = new System.Data.SqlClient.SqlConnection("Data Source=" + ServerName + ";Initial Catalog=" + DatabaseName + ";Integrated Security=SSPI; Connect Timeout=120");

SqlDataAdapter adapter = new SqlDataAdapter("Select * from " + TableName, Connection);
adapter.FillSchema(Table, SchemaType.Source);
adapter.Fill(Table);

DataColumn column = DataTable.Columns[0];

我想做的是：

假设当前column.DataType.Name是"Double"。我希望它变成"Int32"。

我如何实现这一目标？

score 300 · Accepted Answer

在 Datatable 中填充数据后，您无法更改 DataType。但是，您可以克隆数据表，更改列类型并将数据从以前的数据表加载到克隆的表中，如下所示。

DataTable dtCloned = dt.Clone();
dtCloned.Columns[0].DataType = typeof(Int32);
foreach (DataRow row in dt.Rows) 
{
    dtCloned.ImportRow(row);
}

score 33 · Accepted Answer

虽然在DataTable填充之后您不能更改列的类型，但您可以在调用之后更改它FillSchema，但在调用之前Fill。例如，假设第 3 列是您想要从转换为的列double，Int32您可以使用：

adapter.FillSchema(table, SchemaType.Source);
table.Columns[2].DataType = typeof (Int32);
adapter.Fill(table);

score 26 · Accepted Answer

旧帖子，但我想我会权衡一下，使用可以一次将单个列转换为给定类型的 DataTable 扩展：

public static class DataTableExt
{
    public static void ConvertColumnType(this DataTable dt, string columnName, Type newType)
    {
        using (DataColumn dc = new DataColumn(columnName + "_new", newType))
        {
            // Add the new column which has the new type, and move it to the ordinal of the old column
            int ordinal = dt.Columns[columnName].Ordinal;
            dt.Columns.Add(dc);
            dc.SetOrdinal(ordinal);

            // Get and convert the values of the old column, and insert them into the new
            foreach (DataRow dr in dt.Rows)
                dr[dc.ColumnName] = Convert.ChangeType(dr[columnName], newType);

            // Remove the old column
            dt.Columns.Remove(columnName);

            // Give the new column the old column's name
            dc.ColumnName = columnName;
        }
    }
}

然后可以这样调用它：

MyTable.ConvertColumnType("MyColumnName", typeof(int));

当然可以使用您想要的任何类型，只要列中的每个值实际上都可以转换为新类型。

score 12 · Accepted Answer

我采取了一些不同的方法。我需要从 OA 日期格式的 excel 导入中解析日期时间。这种方法很简单，可以从……本质上构建，

添加所需类型的列
翻阅转换值的行

删除原始列并重命名为新列以匹配旧列

private void ChangeColumnType(System.Data.DataTable dt, string p, Type type){
        dt.Columns.Add(p + "_new", type);
        foreach (System.Data.DataRow dr in dt.Rows)
        {   // Will need switch Case for others if Date is not the only one.
            dr[p + "_new"] =DateTime.FromOADate(double.Parse(dr[p].ToString())); // dr[p].ToString();
        }
        dt.Columns.Remove(p);
        dt.Columns[p + "_new"].ColumnName = p;
    }

score 10 · Accepted Answer

10

还考虑更改返回类型：

select cast(columnName as int) columnName from table

于 2013-07-02T19:49:53.567 回答

score 8 · Accepted Answer

Dim tblReady1 As DataTable = tblReady.Clone()

'' convert all the columns type to String 
For Each col As DataColumn In tblReady1.Columns
  col.DataType = GetType(String)
Next

tblReady1.Load(tblReady.CreateDataReader)

score 5 · Accepted Answer

如果您只想更改一个列。例如从字符串更改为 int32，您可以使用表达式属性：

DataColumn col = new DataColumn("col_int" , typeof(int));
table.Columns.Add(col);
col.Expression = "table_exist_col_string"; // digit string convert to int

score 4 · Accepted Answer

一旦 aDataTable被填充，您就不能更改列的类型。

在这种情况下，您最好的选择是在填充之前添加一Int32列：DataTable

dataTable = new DataTable("Contact");
dataColumn = new DataColumn("Id");
dataColumn.DataType = typeof(Int32);
dataTable.Columns.Add(dataColumn);

然后您可以将原始表中的数据克隆到新表中：

DataTable dataTableClone = dataTable.Clone();

这是一个包含更多细节的帖子。

score 4 · Accepted Answer

我将 Mark 解决方案的效率——因此我不必.Clone对整个 DataTable——与泛型和可扩展性相结合，因此我可以定义自己的转换函数。这就是我最终的结果：

/// <summary>
///     Converts a column in a DataTable to another type using a user-defined converter function.
/// </summary>
/// <param name="dt">The source table.</param>
/// <param name="columnName">The name of the column to convert.</param>
/// <param name="valueConverter">Converter function that converts existing values to the new type.</param>
/// <typeparam name="TTargetType">The target column type.</typeparam>
public static void ConvertColumnTypeTo<TTargetType>(this DataTable dt, string columnName, Func<object, TTargetType> valueConverter)
{
    var newType = typeof(TTargetType);

    DataColumn dc = new DataColumn(columnName + "_new", newType);

    // Add the new column which has the new type, and move it to the ordinal of the old column
    int ordinal = dt.Columns[columnName].Ordinal;
    dt.Columns.Add(dc);
    dc.SetOrdinal(ordinal);

    // Get and convert the values of the old column, and insert them into the new
    foreach (DataRow dr in dt.Rows)
    {
        dr[dc.ColumnName] = valueConverter(dr[columnName]);
    }

    // Remove the old column
    dt.Columns.Remove(columnName);

    // Give the new column the old column's name
    dc.ColumnName = columnName;
}

这样，使用起来更加直接，同时也可以自定义：

DataTable someDt = CreateSomeDataTable();
// Assume ColumnName is an int column which we want to convert to a string one.
someDt.ConvertColumnTypeTo<string>('ColumnName', raw => raw.ToString());

score 2 · Accepted Answer

我创建了一个扩展函数，它允许更改 DataTable 的列类型。它不是克隆整个表并导入所有数据，而是克隆列，解析值，然后删除原始数据。

    /// <summary>
    /// Changes the datatype of a column. More specifically it creates a new one and transfers the data to it
    /// </summary>
    /// <param name="column">The source column</param>
    /// <param name="type">The target type</param>
    /// <param name="parser">A lambda function for converting the value</param>
    public static void ChangeType(this DataColumn column, Type type, Func<object, object> parser)
    {
        //no table? just switch the type
        if (column.Table == null)
        {
            column.DataType = type;
            return;
        }

        //clone our table
        DataTable clonedtable = column.Table.Clone();

        //get our cloned column
        DataColumn clonedcolumn = clonedtable.Columns[column.ColumnName];

        //remove from our cloned table
        clonedtable.Columns.Remove(clonedcolumn);

        //change the data type
        clonedcolumn.DataType = type;

        //change our name
        clonedcolumn.ColumnName = Guid.NewGuid().ToString();

        //add our cloned column
        column.Table.Columns.Add(clonedcolumn);

        //interpret our rows
        foreach (DataRow drRow in column.Table.Rows)
        {
            drRow[clonedcolumn] = parser(drRow[column]);
        }

        //remove our original column
        column.Table.Columns.Remove(column);

        //change our name
        clonedcolumn.ColumnName = column.ColumnName;
    }
}

你可以像这样使用它：

List<DataColumn> lsColumns = dtData.Columns
    .Cast<DataColumn>()
    .Where(i => i.DataType == typeof(decimal))
    .ToList()

//loop through each of our decimal columns
foreach(DataColumn column in lsColumns)
{
    //change to double
    column.ChangeType(typeof(double),(value) =>
    {
        double output = 0;
        double.TryParse(value.ToString(), out output);
        return output;  
    });
}

上面的代码将所有小数列更改为双精度。

score 1 · Accepted Answer

DataTable DT = ...
// Rename column to OLD:
DT.Columns["ID"].ColumnName = "ID_OLD";
// Add column with new type:
DT.Columns.Add( "ID", typeof(int) );
// copy data from old column to new column with new type:
foreach( DataRow DR in DT.Rows )
{ DR["ID"] = Convert.ToInt32( DR["ID_OLD"] ); }
// remove "OLD" column
DT.Columns.Remove( "ID_OLD" );

score 0 · Accepted Answer

Puedes agregar una columna con tipo de dato distinto , luego copiar los datos y eliminar la columna anterior

TB.Columns.Add("columna1", GetType(Integer))    
TB.Select("id=id").ToList().ForEach(Sub(row) row("columna1") = row("columna2"))    
TB.Columns.Remove("columna2")

c# - 如何更改数据表中数据列的数据类型？

12 回答 12

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