C# Convert.ToString()与.ToString()的区别，

下面的是Convert的源码：

public static class Convert
{
......
public static string ToString(object value)
{
    return ToString(value, null);
}
......
public static string ToString(object value, IFormatProvider provider)
{
    //如果实现了IConvertible接口，自动调用IConvertible接口的实现方法
    IConvertible convertible = value as IConvertible;
    if (convertible != null)
    {
        return convertible.ToString(provider);
    }
    //如果实现了IFormattable接口，自动调用IFormattable接口的实现方法
    IFormattable formattable = value as IFormattable;
    if (formattable != null)
    {
        return formattable.ToString(null, provider);
    }
    //如果不为空，自动调用ToString方法
    if (value != null)
    {
        return value.ToString();
    }
    //如果为空，返回string.Empty
    return string.Empty;
}
......
}

下面是测试代码：

using System;

namespace ConvertTest
{
    public class FormattableObject : IFormattable
    {
        public string ToString(string format, IFormatProvider formatProvider)
        {
            return "This is a formattable string.";
        }
    }

    public class CommonObject
    {
        public override string ToString()
        {
            return "This is a commonObject.";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            FormattableObject formatObject = new FormattableObject();
            Console.WriteLine(Convert.ToString(formatObject));
            Console.WriteLine(formatObject.ToString());
            CommonObject commonObject = new CommonObject();
            Console.WriteLine(Convert.ToString(commonObject));
            Console.WriteLine(commonObject.ToString());
            commonObject = null;
            Console.WriteLine(Convert.ToString(commonObject) == string.Empty);
            Console.ReadKey();
        }
    }
}

输出结果如下：

This is a formattable string.
Test.FormattableObject
This is a commonObject.
This is a commonObject.
True
オブジェクト参照がオブジェクト インスタンスに設定されていません。