在C#中，字串string還是一種不可變的引用型別，在對字串進行操作修改後，原有的字串記憶體資料會被回收，程式碼所拿到的其實是一個新的字串物件。在需要對字串進行大規模的修改時，會造成GC效能的損耗，C#的標準庫中提供了一個StringBuilder類用來彌補string的這個缺點。
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//Unity的API
[DefaultMember("Chars")]
public sealed class StringBuilder : ISerializable
{
public StringBuilder();
public StringBuilder(int capacity);
public StringBuilder(string value);
public StringBuilder(int capacity, int maxCapacity);
public StringBuilder(string value, int capacity);
public StringBuilder(string value, int startIndex, int length, int capacity);
public char this[int index] { get; set; }
public int Capacity { get; set; }
public int Length { get; set; }
public int MaxCapacity { get; }
public StringBuilder Append(char value, int repeatCount);
[CLSCompliant(false)]
public StringBuilder Append(char* value, int valueCount);
public StringBuilder Append(byte value);
public StringBuilder Append(bool value);
[CLSCompliant(false)]
public StringBuilder Append(ulong value);
[CLSCompliant(false)]
public StringBuilder Append(uint value);
[CLSCompliant(false)]
public StringBuilder Append(ushort value);
public StringBuilder Append(char value);
public StringBuilder Append(StringBuilder value);
public StringBuilder Append(string value, int startIndex, int count);
public StringBuilder Append(string value);
public StringBuilder Append(StringBuilder value, int startIndex, int count);
[CLSCompliant(false)]
public StringBuilder Append(sbyte value);
public StringBuilder Append(ReadOnlySpan<char> value);
public StringBuilder Append(object value);
public StringBuilder Append(long value);
public StringBuilder Append(int value);
public StringBuilder Append(short value);
public StringBuilder Append(double value);
public StringBuilder Append(char[] value);
public StringBuilder Append(char[] value, int startIndex, int charCount);
public StringBuilder Append(float value);
public StringBuilder Append(decimal value);
public StringBuilder AppendFormat(string format, params object[] args);
public StringBuilder AppendFormat(string format, object arg0, object arg1, object arg2);
public StringBuilder AppendFormat(IFormatProvider provider, string format, object arg0, object arg1, object arg2);
public StringBuilder AppendFormat(string format, object arg0);
public StringBuilder AppendFormat(IFormatProvider provider, string format, params object[] args);
public StringBuilder AppendFormat(IFormatProvider provider, string format, object arg0, object arg1);
public StringBuilder AppendFormat(IFormatProvider provider, string format, object arg0);
public StringBuilder AppendFormat(string format, object arg0, object arg1);
public StringBuilder AppendJoin<T>(string separator, IEnumerable<T> values);
public StringBuilder AppendJoin(string separator, params string[] values);
public StringBuilder AppendJoin(string separator, params object[] values);
public StringBuilder AppendJoin(char separator, params object[] values);
public StringBuilder AppendJoin(char separator, params string[] values);
public StringBuilder AppendJoin<T>(char separator, IEnumerable<T> values);
public StringBuilder AppendLine();
public StringBuilder AppendLine(string value);
public StringBuilder Clear();
public void CopyTo(int sourceIndex, char[] destination, int destinationIndex, int count);
public void CopyTo(int sourceIndex, Span<char> destination, int count);
public int EnsureCapacity(int capacity);
public bool Equals(StringBuilder sb);
public bool Equals(ReadOnlySpan<char> span);
[CLSCompliant(false)]
public StringBuilder Insert(int index, ulong value);
[CLSCompliant(false)]
public StringBuilder Insert(int index, uint value);
[CLSCompliant(false)]
public StringBuilder Insert(int index, ushort value);
public StringBuilder Insert(int index, string value, int count);
public StringBuilder Insert(int index, float value);
[CLSCompliant(false)]
public StringBuilder Insert(int index, sbyte value);
public StringBuilder Insert(int index, ReadOnlySpan<char> value);
public StringBuilder Insert(int index, string value);
public StringBuilder Insert(int index, int value);
public StringBuilder Insert(int index, bool value);
public StringBuilder Insert(int index, byte value);
public StringBuilder Insert(int index, long value);
public StringBuilder Insert(int index, char value);
public StringBuilder Insert(int index, object value);
public StringBuilder Insert(int index, char[] value, int startIndex, int charCount);
public StringBuilder Insert(int index, decimal value);
public StringBuilder Insert(int index, double value);
public StringBuilder Insert(int index, short value);
public StringBuilder Insert(int index, char[] value);
public StringBuilder Remove(int startIndex, int length);
public StringBuilder Replace(char oldChar, char newChar, int startIndex, int count);
public StringBuilder Replace(string oldValue, string newValue);
public StringBuilder Replace(string oldValue, string newValue, int startIndex, int count);
public StringBuilder Replace(char oldChar, char newChar);
public override string ToString();
public string ToString(int startIndex, int length);
}
從上面的API我們可以看出來StringBuilder具有值型別相容性、增刪改查、文字格式化等功能。當對StringBuilder修改後，返回的是原來的StringBuilder物件，重複使用StringBuilder例項，可以減少不必要的記憶體分配。
StringBuilder的使用也十分簡單，一個簡單的例子：將一個字串陣列的元素拼接成一個字串，並以/分隔
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List<string> stringArr = new List<string> { "A", "B", "C" };
StringBuilder sb = new StringBuilder();
for (int i = 0; i < stringArr.Count; i++)
{
sb.Append(stringArr[i] + "/");
}
string result = sb.ToString();