Word1252_7bit
是结构
Key
为Int32
如果找不到值,我如何测试null?
没有w.Key == -1但我不知道测试没有返回值。
最后一个Debug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。寻找在独特的Int32上搜索速度。
不确定它是否有所不同但Key是唯一的并且我使用Key来覆盖GetHashCode(),并且为了节省空间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;
}
}
答案 0 :(得分:3)
FirstOrDefault<T>
将返回找到的第一个项目或默认值T
。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
关键字:
if (word1252_7bit.Equals(default(Word1252_7bit)))
Debug.WriteLine("not found");
注意:您可能必须写自己覆盖Equals
以获得预期的结果。
通常,您将无法区分未找到的项目与找到的项目但是等于默认值。你可以这样做:
int foundAt = Words7bit.FindIndex(w => w.Key == -1);
if (foundAt == -1)
Debug.WriteLine("not found");
else
word1252_7bit = Words7bit[foundAt];
答案 1 :(得分:1)
要获得更多控制权,您可以定义自己的Default
值,例如使用类似的东西:
public static T FirstOrDefault<T>(this IEnumerable<T> sequence, T defaultValue)
{
foreach (var element in sequence)
return element;
return defaultValue; // default(T);
// return sequence.Any() ? sequence.First() : defaultValue;
}
然后像
一样使用它word1252_7bit = Words7bit.Where(w => w.Key == 1000)
.FirstOrDefault(Word1252_7bit.Default);
其中“Default”是为结构定义的默认实例。有点像...
public static readonly Word1252_7bit Default = new Word1252_7bit
{
Key = Int32.MinValue,
Value = "default",
};
编辑:改进的FirstOrDefault
代码(基于原始实现)