Question

我打算创建自己的字符串类，但是它将提供最大长度。我们称之为“ lString”。我想像代码中的“ string”类一样使用“ lString”。但是我可以为其设置最大长度。

例如，应构建以下代码：

// 1- No maxlength provided, so the object will be created.
lString mylString1 = "0123456789";
// 2- maxlength provided, so it will be checked, and then created.
lString mylString2 = new lString("0123456789", 10);
// 3- This time only maxlength provided, so it will be a string object with maxLength.
lString mylString3 = new lString(20);

// At the end, I should be able to use it like a regular strings:
mylString3 = mylString1 + mylString2;

// Below should throw exception at RunTime, because it will be over 20)
mylString3 = mylString1 + mylString2 + mylString1 + mylString2;

Answer 1

实现一个基本的字符串类非常简单，该类具有对普通字符串的隐式运算符：

public class LimitedString
{
    private readonly string value;
    private readonly long maxLength;

    public LimitedString(string value, long maxLength = long.MaxValue)
    {
        if(value != null && value.Length > maxLength)
            throw new InvalidOperationException("Value is longer than max length");
        this.value = value;
        this.maxLength = maxLength;
    }

    public override string ToString()
    {
        return value;
    }

    public override int GetHashCode()
    {
        return value.GetHashCode();
    }

    public override bool Equals(object o)
    {
        if(o is LimitedString)
            return value == ((LimitedString)o).value;
        return false;
    }

    public static implicit operator LimitedString(string str)
    {
        return new LimitedString(str);
    }

    public static implicit operator String(LimitedString ls)
    {
        return ls.value;
    }        
}

然后您的前两个案例按预期工作：

LimitedString myString1 = "0123456789";
LimitedString myString2 = new LimitedString("0123456789", 10);

但是，使第三个示例工作的唯一方法是这样的：

LimitedString myString3 = new LimitedString(myString1 + myString2 + myString1 + myString2, 20); // Throws exception

重新分配该值后，最大长度的规范将丢失，因此您不能这样做：

LimitedString myString3 = new LimitedString(20); // This is fine - you could have a constructor that just takes the max length.
myString3 = myString1 + myString2 + myString1 + myString2; // but here you're re-assigning.

实时示例：https://rextester.com/KVBBQT19360

如何使用MaxLength设计String类？

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