c# - LINQ FirstOrDefault 测试 null

Question

Word1252_7bit 是一个结构体
Key 是 Int32

如果找不到值，我如何测试是否为 null？

没有 w.Key == -1 但我不知道测试是否返回值。

最后的调试行抛出异常。

List<Word1252_7bit> Words7bit = GetWord1252_7bit();

Word1252_7bit word1252_7bit ;

word1252_7bit = Words7bit.FirstOrDefault(w => w.Key == 1000);
Debug.WriteLine(word1252_7bit.Key.ToString() + " " + word1252_7bit.Value);

word1252_7bit = Words7bit.FirstOrDefault(w => w.Key == -1);
//if (word1252_7bit == null) Debug.WriteLine("word1252_7bit == null");
Debug.WriteLine( word1252_7bit.Key.ToString() + " " + word1252_7bit.Value ) ;

如果我应该使用 FirstOrDefault 以外的东西，请告诉我。在唯一的 Int32 上寻找速度搜索。

不确定它是否有所不同，但 Key 是唯一的，我使用 Key 来覆盖 GetHashCode()，为了节省空间，Key 实际上是 UInt32 的一部分

public Int32 Key
{
  get
  {
    return (Int32)( pack[0] & ( (1<<25) - 1 ) ) ;
  }
}

public struct Word1252_7bit : iWord
{
    // this maps 128 values to "Windows-1252" 
    // this is not ASCII 
    // this is SQL char 8bit normalized to FormD, remove control chars, remove redactions, and cast to lower - 129 - just have to cheat on 1
    private static byte[] Win1252_128to256 = new byte[] {  
             32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63
           , 64, 91, 92, 93, 94, 95, 96, 97, 98, 99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121
           ,122,123,124,125,126,128,130,131,132,133,134,135,137,139,145,146,147,148,149,150,151,152,153,155,156,160,161,162,163,164,165,166
           ,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,215,223,230,240,247,248,254 };
    private static Encoding win1252 = Encoding.GetEncoding("Windows-1252");
    private UInt32[] pack;
    public Int32 Key { get { return (Int32)(pack[0] & ((1 << 25) - 1)); } }
    public override bool Equals(Object obj)
    {
        // Check for null values and compare run-time types.
        if (obj == null) return false;
        if (!(obj is Word1252_7bit)) return false;
        Word1252_7bit comp = (Word1252_7bit)obj;
        if (comp.pack == null) return false;
        if (comp.pack.Count() == 0) return false;
        return (comp.Key == this.Key);
    }
    public override int GetHashCode()
    {
        return Key;
    }
    public byte[] Bytes
    {
        get
        {
            byte b;              
            List<byte> bytes = new List<byte>(((pack.Length - 1) * 4) + 1);

            b = (byte)((pack[0] >> 25) & ((1 << 7) - 1));
            bytes.Add(Win1252_128to256[b]);

            if (pack.Length > 1)
            {                       
                UInt32 cur32;
                byte bits4 = 0;
                byte bits3 = 0;
                for (int i = 1; i < pack.Length; i++)
                {
                    cur32 = pack[i];

                    if ((i-1) % 2 == 0)
                    {
                        bits4 = (byte)((cur32 >> 28) & ((1 << 4) - 1));
                    }
                    else
                    {   // pick up that odd i7
                        bits3 = (Byte)((cur32 >> 28) & ((1 << 3) - 1));
                        b = (byte)((UInt32)bits3 | ((UInt32)bits4 << 3));
                        if (b == 0) break;
                        bytes.Add(Win1252_128to256[b]);
                    }

                    b = (byte)(cur32         & ((1 << 7) - 1));
                    if (b == 0) break;
                    bytes.Add(Win1252_128to256[b]);

                    b = (byte)((cur32 >>  7) & ((1 << 7) - 1));
                    if (b == 0) break;
                    bytes.Add(Win1252_128to256[b]);

                    b = (byte)((cur32 >> 14) & ((1 << 7) - 1));
                    if (b == 0) break;
                    bytes.Add(Win1252_128to256[b]);

                    b = (byte)((cur32 >> 21) & ((1 << 7) - 1));
                    if (b == 0) break;
                    bytes.Add(Win1252_128to256[b]);

                    //Debug.WriteLine(win1252.GetString(bytes.ToArray()));   
                }
            }
            return bytes.ToArray();
        }
    }
    public String Value
    {
        get
        {
            return win1252.GetString(Bytes);
        }
    }
    public Int32 Lenght { get { return Bytes.Count(); } }
    public Word1252_7bit(UInt32[] Pack)
    {
        if(Pack == null) throw new IndexOutOfRangeException();
        if (Pack.Length == 0) throw new IndexOutOfRangeException();
        pack = Pack;
    }
}

Answer 1

FirstOrDefault<T>将返回找到的第一个项目或默认值 T如果没有找到。对于引用类型，这是 null ，但这对于值类型是不同的。例如，整数的默认值为 0 .

您可以使用 default 关键词:

if (word1252_7bit.Equals(default(Word1252_7bit)))
    Debug.WriteLine("not found");

^{注意:您可能必须编写自己对 Equals 的覆盖以获得您期望的结果。}

通常，您无法区分未找到的项目和已找到但等于默认值的项目。您可以这样做:

int foundAt = Words7bit.FindIndex(w => w.Key == -1);
if (foundAt == -1)
    Debug.WriteLine("not found");
else
    word1252_7bit = Words7bit[foundAt];