DataGridView单元格中的自定义控件在编辑时引发FormatException

Question

我创建了一个自定义控件来编辑DataGridView单元格中的自定义值。我在这里举一个例子：如何：Windows窗体中的主机控件DataGridView单元格

我有一个自定义数据类，它是绑定到DataGridView的对象列表的成员。

internal class CustomValue
{
   // Some stuff is here.
}

我创建了一个自定义控件来编辑实现IDataGridViewEditingControl接口的单元格中的值。

internal partial class CustomValueEditControl : UserControl, IDataGridViewEditingControl
{
        DataGridView m_dataGridView;
        private bool m_valueChanged = false;
        int m_rowIndex;
        private CustomValue m_value;
        public CustomValue Value
        {
            get
            {
                return m_value;
            }
            set
            {
                m_value = value;
            }
        }
        public void ApplyCellStyleToEditingControl(DataGridViewCellStyle dataGridViewCellStyle)
        {
        }
        public DataGridView EditingControlDataGridView
        {
            get
            {
                return m_dataGridView;
            }
            set
            {
                m_dataGridView = value;
            }
        }
        public object EditingControlFormattedValue
        {
            get
            {
                return this.Value.ToString();
            }
            set
            {
                if (value is String)
                {
                    CustomValue val;
                    if (CustomValue.TryParse((String)value, out val))
                    {
                        this.Value = val;
                    }
                    else
                    {
                        this.Value = new CustomValue();
                    }
                }
            }
        }
        public int EditingControlRowIndex
        {
            get
            {
                return m_rowIndex;
            }
            set
            {
                m_rowIndex = value;
            }
        }
        public bool EditingControlValueChanged
        {
            get
            {
                return m_valueChanged;
            }
            set
            {
                m_valueChanged = value;
            }
        }
        public bool EditingControlWantsInputKey(Keys keyData, bool dataGridViewWantsInputKey)
        {
            switch (keyData & Keys.KeyCode)
            {
                case Keys.Up:
                case Keys.Down:
                    return true;
                default:
                    return !dataGridViewWantsInputKey;
            }
        }
        public Cursor EditingPanelCursor
        {
            get
            {
                return base.Cursor;
            }
        }
        public object GetEditingControlFormattedValue(DataGridViewDataErrorContexts context)
        {
            return EditingControlFormattedValue;
        }
        public void PrepareEditingControlForEdit(bool selectAll)
        {
        }
        public bool RepositionEditingControlOnValueChange
        {
            get
            {
                return false;
            }
        }
}

我设置了一个自定义DataGridViewCell，它是从DataGridViewTextBoxCell派生而来的，因此该单元格在编辑之前只显示自定义值的字符串表示形式。

internal class CustomValueCell : DataGridViewTextBoxCell
{
    public override void InitializeEditingControl(int rowIndex, object initialFormattedValue, DataGridViewCellStyle dataGridViewCellStyle)
    {
        base.InitializeEditingControl(rowIndex, initialFormattedValue, dataGridViewCellStyle);
        CustomValueEditControl customValueEditControl = DataGridView.EditingControl as CustomValueEditControl;
        if (this.Value == null)
        {
            customValueEditControl.Value = (CustomValue)this.DefaultNewRowValue;
        }
        else
        {
            customValueEditControl.Value = (CustomValue)this.Value;
        }
    }
    public override Type EditType
    {
        get
        {
            return typeof(CustomValueEditControl);
        }
    }
    public override Type ValueType
    {
        get
        {
            return typeof(CustomValue);
        }
    }
    public override Type FormattedValueType
    {
        get
        {
            return typeof(string);
        }
    }
    public override object DefaultNewRowValue
    {
        get
        {
            return new CustomValue();
        }
    }
}

我已经创建了一个自定义DataGridViewColumn。

internal class CustomValueColumn : DataGridViewColumn
{
    public CustomValueColumn()
        : base(new CustomValueCell())
    {
    }
    public override object Clone()
    {
        CustomValueColumn clone = (CustomValueColumn)base.Clone();
        return clone;
    }
    public override DataGridViewCell CellTemplate
    {
        get
        {
            return base.CellTemplate;
        }
        set
        {
            // Ensure that the cell used for the template is a CustomValueCell.
            if (value != null && !value.GetType().IsAssignableFrom(typeof(CustomValueCell)))
            {
                throw new InvalidCastException("Must be a CustomValueCell");
            }
            base.CellTemplate = value;
        }
    }
}

该单元格正确显示了自定义类型的值，我可以使用自定义控件来编辑该值。离开编辑单元格时，我在System.Convert.DefaultToType（IConvertable value，Type targetType，IFormatProvider提供程序）处得到System.FormatException：从"System.String"到"CustomValue"的无效强制转换。。。

它在什么时候尝试将字符串强制转换为我的自定义值？这不应该由CustomValueEditControl.GetEditingControlFormattedValue处理吗？

Answer 1

所以我最终通过实现TypeConverter类来解决这个问题。

internal class CustomValueTypeConverter : TypeConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        if (sourceType == typeof(string))
        {
            return true;
        }
        return base.CanConvertFrom(context, sourceType);
    }
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        if (destinationType == typeof(string))
        {
            return true;
        }
        return base.CanConvertTo(context, destinationType);
    }
    public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
    {
        if (value is string)
        {
            CustomValue customValue;
            if (CustomValue.TryParse((string)value, out customValue))
            {
                return customValue;
            }
        }
        return base.ConvertFrom(context, culture, value);
    }
    public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)
    {
        if (destinationType == typeof(string))
        {
            value.ToString();
        }
        return base.ConvertTo(context, culture, value, destinationType);
    }
}

然后我不得不用TypeConverterAttribute来修饰CustomValue类。

[TypeConverter(typeof(CustomValueTypeConverter))]
internal class CustomValue
{
  // Some stuff here.
}

Answer 2

它在什么时候尝试将字符串强制转换为我的自定义值？这不应该由CustomValueEditControl.GetEditingControlFormattedValue处理吗？

没错。但是您所基于的样本处理不正确。你应该使用这样的东西：

public object GetEditingControlFormattedValue(DataGridViewDataErrorContexts context)
{
    if ((context & DataGridViewDataErrorContexts.Parsing) != 0)
    {
        // Here you should not return string, but your value
        return Value;
    }
    return EditingControlFormattedValue;
}

如果返回string（EditingControlFormattedValue属性执行此操作），则DataGridView将尝试将其转换为CustomValue对象。这个例子之所以有效，是因为DateTime有关联的TypeConverter，而您的类显然没有。