带接口的TypeConverter

Question

我在将C#中的属性实现为设计器类型时遇到了一些小问题。基本上，我有多个实现接口的具体类型。我想在设计时选择哪种混凝土类型。

using System;
using System.ComponentModel;
using System.Drawing;

/// <summary>
/// implement a summarizable type
/// </summary>
public interface ISummarizable
{
    /// <summary>
    /// Summarize along rows
    /// </summary>
    /// <param name="rawdata"></param>
    /// <returns></returns>
    double[] Summarize(double[,] rawdata);
}

/// <summary>
/// calculate by mean value
/// </summary>
public class MeanSummary : ISummarizable
{
    public double[] Summarize(double[,] rawdata)
    {
        double[] result = new double[rawdata.GetLength(0)];
        for (int n = 0; n < result.Length; n++)
        {
            double value = 0;
            for (int j = 0; n < rawdata.GetLength(1); j++)
                value += rawdata[n, j] / rawdata.GetLength(1);
            result[n] = value;
        }
        return result;
    }
}

/// <summary>
/// Calculate by max value
/// </summary>
public class MaxSummary : ISummarizable
{
    public double[] Summarize(double[,] rawdata)
    {
        double[] result = new double[rawdata.GetLength(0)];
        for (int n = 0; n < result.Length; n++)
        {
            double value = double.MinValue;     // guaranteed to change at least once
            for (int j = 0; n < rawdata.GetLength(1); j++)
                value = Math.Max(value, rawdata[n, j]);
            result[n] = value;
        }
        return result;
    }
}

/// <summary>
/// type converter
/// </summary>
public class SummaryTypeConverter : System.ComponentModel.TypeListConverter
{
    public override bool CanConvertFrom(System.ComponentModel.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(System.ComponentModel.ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
    {
        if (value is string)
            return value.ToString();
        return base.ConvertFrom(context, culture, value);
    }

    public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)
    {
        if (destinationType == typeof(string))
        {
            if(value is ISummarizable)
            {
                return value.GetType().Name;
            }
        }
        return base.ConvertTo(context, culture, value, destinationType);
    }


    /// <summary>
    /// constructor w/ initializer
    /// </summary>
    public SummaryTypeConverter() :
        base(new Type[] { typeof(MeanSummary), typeof(MaxSummary) })
    {
    }
}

public class MyControl : System.Windows.Forms.Control
{
    protected ISummarizable _summary = new MaxSummary();

    [TypeConverter(typeof(SummaryTypeConverter))]
    public ISummarizable Summary { get { return _summary; } set { _summary = value; } }


}

在属性查看器中，我为“汇总”属性提供了一个不错的下拉列表，其中包含了我的两种具体类型。但是在选择一个时，错误是

Object of type System.String cannot be converted to type Example.ISummarizable

我尝试在ConvertFrom和ConvertTo中实现ConvertFrom和ConvertTo，但没有成功，并尝试使用第二个Visual实例对其进行调试，但我甚至无法捕获此错误。有什么想法吗？

Answer 1

在您的ConvertFrom实现中，在这种情况下，您会收到一个有意义的字符串(例如。)，您需要返回实际类型的实例，而不是调用基方法(正如Carsten所说)。

您可以尝试使用反射(查找具有该名称的类型，检查它是否为ISummarizable，调用构造函数)或使用开关语句。

通过反射，您的代码将是：

public override object ConvertFrom(System.ComponentModel.ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
{
    if (value is string)
    {
        var source = (string)value;
        var sourceType = Type.GetType(source);

        if (sourceType != null && typeof(ISummarizable).IsAssignableFrom(sourceType))
        {
            var constructor = sourceType.GetConstructor(Type.EmptyTypes);
            var sourceInstance = constructor.Invoke(new object[0]);

            return sourceInstance;
        }
    }

    return base.ConvertFrom(context, culture, value);
}