StringBuilder.Append()方法比使用+操作符要好得多。但我发现，当执行1000个或更少的连接时，String.Join()甚至比StringBuilder更有效。

StringBuilder sb = new StringBuilder();
sb.Append(someString);

String的唯一问题。Join是指必须使用公共分隔符连接字符串。

编辑:正如@ryanversaw指出的，可以将分隔符设置为string.Empty。

string key = String.Join("_", new String[] 
{ "Customers_Contacts", customerID, database, SessionID });

如果你在循环中操作，StringBuilder可能是最好的选择;它节省了定期创建新字符串的开销。在只运行一次的代码中，字符串。Concat可能没问题。

然而，Rico Mariani(。NET优化大师)做了一个测试，他在测试的最后说，在大多数情况下，他建议使用String.Format。

对于只有两个字符串，您肯定不希望使用StringBuilder。如果超过某个阈值，StringBuilder开销将小于分配多个字符串的开销。

所以，对于超过2-3个字符串，使用DannySmurf的代码。否则，只需使用+运算符。

我已经测试了本页中的所有方法，最后我开发出了最快且内存成本更低的解决方案。

注:在Framework 4.8中测试

 [MemoryDiagnoser]
public class StringConcatSimple
{
    private string
        title = "Mr.", firstName = "David", middleName = "Patrick", lastName = "Callan";

    [Benchmark]
    public string FastConcat()
    {
        return FastConcat(
            title, " ", 
            firstName, " ",
            middleName, " ", 
            lastName);
    }

    [Benchmark]
    public string StringBuilder()
    {
        var stringBuilder =
            new StringBuilder();

        return stringBuilder
            .Append(title).Append(' ')
            .Append(firstName).Append(' ')
            .Append(middleName).Append(' ')
            .Append(lastName).ToString();
    }

    [Benchmark]
    public string StringBuilderExact24()
    {
        var stringBuilder =
            new StringBuilder(24);

        return stringBuilder
            .Append(title).Append(' ')
            .Append(firstName).Append(' ')
            .Append(middleName).Append(' ')
            .Append(lastName).ToString();
    }

    [Benchmark]
    public string StringBuilderEstimate100()
    {
        var stringBuilder =
            new StringBuilder(100);

        return stringBuilder
            .Append(title).Append(' ')
            .Append(firstName).Append(' ')
            .Append(middleName).Append(' ')
            .Append(lastName).ToString();
    }

    [Benchmark]
    public string StringPlus()
    {
        return title + ' ' + firstName + ' ' +
            middleName + ' ' + lastName;
    }

    [Benchmark]
    public string StringFormat()
    {
        return string.Format("{0} {1} {2} {3}",
            title, firstName, middleName, lastName);
    }

    [Benchmark]
    public string StringInterpolation()
    {
        return
        $"{title} {firstName} {middleName} {lastName}";
    }

    [Benchmark]
    public string StringJoin()
    {
        return string.Join(" ", title, firstName,
            middleName, lastName);
    }

    [Benchmark]
    public string StringConcat()
    {
        return string.
            Concat(new String[]
            { title, " ", firstName, " ",
                middleName, " ", lastName });
    }
}

是的，它使用不安全

public static unsafe string FastConcat(string str1, string str2, string str3, string str4, string str5, string str6, string str7)
    {
        var capacity = 0;

        var str1Length = 0;
        var str2Length = 0;
        var str3Length = 0;
        var str4Length = 0;
        var str5Length = 0;
        var str6Length = 0;
        var str7Length = 0;

        if (str1 != null)
        {
            str1Length = str1.Length;
            capacity = str1Length;
        }

        if (str2 != null)
        {
            str2Length = str2.Length;
            capacity += str2Length;
        }

        if (str3 != null)
        {
            str3Length = str3.Length;
            capacity += str3Length;
        }

        if (str4 != null)
        {
            str4Length = str4.Length;
            capacity += str4Length;
        }

        if (str5 != null)
        {
            str5Length = str5.Length;
            capacity += str5Length;
        }

        if (str6 != null)
        {
            str6Length = str6.Length;
            capacity += str6Length;
        }

        if (str7 != null)
        {
            str7Length = str7.Length;
            capacity += str7Length;
        }


        string result = new string(' ', capacity);

        fixed (char* dest = result)
        {
            var x = dest;

            if (str1Length > 0)
            {
                fixed (char* src = str1)
                {
                    Unsafe.CopyBlock(x, src, (uint)str1Length * 2); 
                    x += str1Length;
                }
            }

            if (str2Length > 0)
            {
                fixed (char* src = str2)
                {
                    Unsafe.CopyBlock(x, src, (uint)str2Length * 2);
                    x += str2Length;
                }
            }

            if (str3Length > 0)
            {
                fixed (char* src = str3)
                {
                    Unsafe.CopyBlock(x, src, (uint)str3Length * 2);
                    x += str3Length;
                }
            }

            if (str4Length > 0)
            {
                fixed (char* src = str4)
                {
                    Unsafe.CopyBlock(x, src, (uint)str4Length * 2);
                    x += str4Length;
                }
            }

            if (str5Length > 0)
            {
                fixed (char* src = str5)
                {
                    Unsafe.CopyBlock(x, src, (uint)str5Length * 2);
                    x += str5Length;
                }
            }

            if (str6Length > 0)
            {
                fixed (char* src = str6)
                {
                    Unsafe.CopyBlock(x, src, (uint)str6Length * 2);
                    x += str6Length;
                }
            }

            if (str7Length > 0)
            {
                fixed (char* src = str7)
                {
                    Unsafe.CopyBlock(x, src, (uint)str7Length * 2);
                }
            }
        }

        return result;
    }

您可以编辑该方法并使其适应您的情况。例如，你可以让它像这样

FastConcat(string str1, string str2, string str3 = null, string str4 = null, string str5 = null, string str6 = null, string str7 = null)

除了其他答案之外，请记住StringBuilder可以被告知要分配的初始内存量。

容量参数定义了当前实例分配的内存中可以存储的最大字符数。它的值被赋给Capacity属性。如果当前实例中存储的字符数量超过了这个容量值，StringBuilder对象就会分配额外的内存来存储这些字符。 如果容量为零，则使用特定于实现的默认容量。

重复添加到未预先分配的StringBuilder可能会导致大量不必要的分配，就像重复连接常规字符串一样。

If you know how long the final string will be, can trivially calculate it, or can make an educated guess about the common case (allocating too much isn't necessarily a bad thing), you should be providing this information to the constructor or the Capacity property. Especially when running performance tests to compare StringBuilder with other methods like String.Concat, which do the same thing internally. Any test you see online which doesn't include StringBuilder pre-allocation in its comparisons is wrong.

如果你无法猜测它的大小，你可能在写一个效用函数它应该有自己的可选参数来控制预分配。

系统。字符串是不可变的。当我们修改一个字符串变量的值时，一个新的内存被分配给新的值，之前的内存分配被释放。系统。StringBuilder被设计为具有可变字符串的概念，其中可以执行各种操作，而无需为修改后的字符串分配单独的内存位置。