結合一個數組中的多個LINQ表達式

Question

我試圖將這樣的函數列表結合起來。

我有這樣的：

Func<int, bool>[] criteria = new Func<int, bool>[3]; 
criteria[0] = i => i % 2 == 0; 
criteria[1] = i => i % 3 == 0; 
criteria[2] = i => i % 5 == 0; 

而且我想這一點：

Func<int, bool>[] predicates = new Func<int, bool>[3]; 
predicates[0] = i => i % 2 == 0; 
predicates[1] = i => i % 2 == 0 && i % 3 == 0; 
predicates[2] = i => i % 2 == 0 && i % 3 == 0 && i % 5 == 0; 

到目前爲止，我有以下代碼：

Expression<Func<int, bool>>[] results = new Expression<Func<int, bool>>[criteria.Length]; 

for (int i = 0; i < criteria.Length; i++) 
{ 
    results[i] = f => true; 
    for (int j = 0; j <= i; j++) 
    { 
     Expression<Func<int, bool>> expr = b => criteria[j](b); 
     var invokedExpr = Expression.Invoke(
      expr, 
      results[i].Parameters.Cast<Expression>()); 
     results[i] = Expression.Lambda<Func<int, bool>>(
      Expression.And(results[i].Body, invokedExpr), 
      results[i].Parameters); 
    } 
} 
var predicates = results.Select(e => e.Compile()).ToArray(); 

Console.WriteLine(predicates[0](6)); // Returns true 
Console.WriteLine(predicates[1](6)); // Returns false 
Console.WriteLine(predicates[2](6)); // Throws an IndexOutOfRangeException 

有誰知道我米做錯了嗎？

Answer 1

這是一種猜測，因爲我不太瞭解這個東西，但是這似乎解決它：

Func<int, bool>[] criteria = new Func<int, bool>[3]; 
criteria[0] = i => i % 2 == 0; 
criteria[1] = i => i % 3 == 0; 
criteria[2] = i => i % 5 == 0; 
Expression<Func<int, bool>>[] results = new Expression<Func<int, bool>>[criteria.Length]; 
for (int i = 0; i < criteria.Length; i++) 
{ 
    results[i] = f => true; 
    for (int j = 0; j <= i; j++) 
    { 
     int ii = i; 
     int jj = j; 
     Expression<Func<int, bool>> expr = b => criteria[jj](b); 
     var invokedExpr = Expression.Invoke(expr, results[ii].Parameters.Cast<Expression>()); 
     results[ii] = Expression.Lambda<Func<int, bool>>(Expression.And(results[ii].Body, invokedExpr), results[ii].Parameters); 
    } 
} 
var predicates = results.Select(e => e.Compile()).ToArray(); 

的關鍵是引入「二」和「JJ」（也許只有一個事項，我沒有嘗試）。我認爲你在一個lambda中捕獲一個可變變量，因此當你最終引用它時，你會看到後來突變的值而不是原始值。

Answer 2

無需在表達式中拉...

Func<int, bool>[] criteria = new Func<int, bool>[3]; 
    criteria[0] = i => i % 2 == 0; 
    criteria[1] = i => i % 3 == 0; 
    criteria[2] = i => i % 5 == 0; 

    Func<int, bool>[] predicates = new Func<int, bool>[3]; 

    predicates[0] = criteria[0]; 
    for (int i = 1; i < criteria.Length; i++) 
    { 
     //need j to be an unchanging int, one for each loop execution. 
     int j = i; 

     predicates[j] = x => predicates[j - 1](x) && criteria[j](x); 
    } 

    Console.WriteLine(predicates[0](6)); //True 
    Console.WriteLine(predicates[1](6)); //True 
    Console.WriteLine(predicates[2](6)); //False