C# 为类和运算符分配默认属性 =

Question

Problem 1:

I have a simple winforms app and I want to DataBind my Person.Name property to a textbox. Name is of type StringField. I originally defined the Name property as String. The data binding works great on value types such as String. I would like the StringField.Value property to be the default property of StringField. I want to see the value of StringField.Value in the textBox rather than the text "FieldApp.StringField".

Problem 2:

I would like to be able to assign a string to a StringField using operator =. This assignment would result in the StringField.Value member being set.

Can this be accomplished?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace FieldApp
{
    public class StringField
    {
        public string Value { get; set; }    
    }

    public class Person
    {

        //private String _Name;
        //public String Name
        //{
        //    get { return _Name; }
        //    set { _Name = value; }
        //}

        //public Person(string name)
        //{
        //    Name = name;
        //}

        private StringField _Name;
        public StringField Name
        {
            get { return _Name; }
            set { _Name = value; }
        }

        public Person(string name)
        {
            Name = new StringField();
            Name.Value = name;
        }
    }

    public partial class FieldAppForm : Form
    {
        Person person = new Person("steve");

        public FieldAppForm()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //our form contains a button1 and textBox1

            //this compiles
            person.Name.Value = "steve";

            //this does not. Is there anyway to accomplish this?
            person.Name = "steve";

            //steve appears in the textbox 
            textBox1.DataBindings.Add("Text", person, "Name.Value");

            //FieldApp.StringField appears in the textbox 
            textBox1.DataBindings.Add("Text", person, "Name");
        }
    }
}

Answer 1

You could create an implicit operator overload. Then you can create StringField from strings like this:

StringField field = "value of new object";
string value=(string)field;

Know that this creates a new StringField object. I wouldn't necessarily advice you to do this.

[System.Diagnostics.DebuggerDisplay("{Value}")]
public class StringField
{
    public string Value { get; set; }
    public static implicit operator StringField(string s)
    {
        return new StringField { Value = s };
    }

    public static explicit operator string(StringField f)
    {
        return f.Value;
    }
    public override string ToString()
    {
        return Value;
    }
}

Answer 2

Re data-binding, for some binding targets ( PropertyGrid , DataGridView , etc), you can do this with a TypeConverter (see below). Unfortunately, this doesn't seem to work with TextBox , so I think your best option is to simply add a shim property (as already suggested):

string NameString
{
   get { return Name.Value; }
   set { Name.Value = value; } // or new blah...
}

(and bind to NameString )

In the past, I have used custom PropertyDescriptor implementations to side-step this, but it isn't worth it just for this.

Anyway, a TypeConverter example (works with PropertyGrid and DataGridView ):

[TypeConverter(typeof(StringFieldConverter))]
public class StringField
{
    public StringField() : this("") { }
    public StringField(string value) { Value = value; }
    public string Value { get; private set; }
}

class StringFieldConverter : TypeConverter
{
    public override bool CanConvertFrom(
        ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string)
            || base.CanConvertFrom(context, sourceType);
    }
    public override object ConvertFrom(
        ITypeDescriptorContext context,
        System.Globalization.CultureInfo culture,
        object value)
    {
        string s = value as string;
        if (s != null) return new StringField(s);
        return base.ConvertFrom(context, culture, value);
    }
    public override bool CanConvertTo(
        ITypeDescriptorContext context, Type destinationType)
    {
        return destinationType == typeof(string)
            || base.CanConvertTo(context, destinationType);
    }
    public override object ConvertTo(
        ITypeDescriptorContext context,
        System.Globalization.CultureInfo culture,
        object value, Type destinationType)
    {
        if (destinationType == typeof(string) && value != null
            && value is StringField)
        {
            return ((StringField)value).Value;
        }
        return base.ConvertTo(context, culture, value, destinationType);
    }
}

Answer 3

You can implement assignment by providing a conversion operator. Given the nature of your class, you should also override the Object methods:

public class StringField {
  public string Value { get; set; }
  public static implicit operator StringField(string value) {
    StringField sf = new StringField();
    sf.Value = value;
    return sf;
  }
  public override string ToString() {
    return Value;
  }
  public override bool Equals(object obj) {
    if (obj == null || !(obj is StringField)) return false;
    return 0 == string.Compare(Value, (obj as StringField).Value);
  }
  public override int GetHashCode() {
    return Value.GetHashCode();
  }
}

Answer 4

You can hade the StringField by mapping the Name property to the Name.Value field internally in your class.

so you can define the Name property like this:

string Name 
{
   get { return _name.Value; }
   set { _name.Value = value; }
}

Here _name is your StringField variable.

Answer 5

The assignment operator cannot be overridden in C#. You could however have a property to do the type conversion for you and expose that one the class