用於重複比較的謂詞表達式函數

Question

我有一個測試幾乎200個案例的謂詞處理程序，每個測試涉及5個可能的比較。我想簡化這些代碼，但是如何在語法上表達這種語法會遇到困難。

public static Expression<Func<OADataConsolidated, bool>> Match(DOTSearchFilter filters) 
{ 
    var predicate = (filters.OrFilters.Count > 0) ? PredicateBuilder.False<OADataConsolidated>() : PredicateBuilder.True<OADataConsolidated>(); 

    foreach (DOTFilter f in filters.AndFilters) 
    { 
     int value = -1; 
     int.TryParse(f.TextValue, out value); 
     switch (f.Type) 
     { 
      case DOTFilter.FilterType.SCO: 
       switch (f.IdValue) 
       { 
        case 4: // GED: Reasoning 
         switch (f.Comp) 
         { 
          case DOTFilter.Comparitor.LessThan: 
           predicate = predicate.And(p => p.ajblGEDR_Mean < value); 
           break; 
          case DOTFilter.Comparitor.EqualOrLess: 
           predicate = predicate.And(p => p.ajblGEDR_Mean <= value); 
           break; 
          case DOTFilter.Comparitor.EqualTo: 
           predicate = predicate.And(p => p.ajblGEDR_Mean == value); 
           break; 
          case DOTFilter.Comparitor.EqualOrGreater: 
           predicate = predicate.And(p => p.ajblGEDR_Mean >= value); 
           break; 
          case DOTFilter.Comparitor.GreaterThan: 
           predicate = predicate.And(p => p.ajblGEDR_Mean > value); 
           break; 
         } 
         break; 
        case 5: // GED: Mathematics 
         switch (f.Comp) 
         { 
          case DOTFilter.Comparitor.LessThan: 
           predicate = predicate.And(p => p.ajblGEDM < value); 
           break; 
          case DOTFilter.Comparitor.EqualOrLess: 
           predicate = predicate.And(p => p.ajblGEDM <= value); 
           break; 
          case DOTFilter.Comparitor.EqualTo: 
           predicate = predicate.And(p => p.ajblGEDM == value); 
           break; 
          case DOTFilter.Comparitor.EqualOrGreater: 
           predicate = predicate.And(p => p.ajblGEDM >= value); 
           break; 
          case DOTFilter.Comparitor.GreaterThan: 
           predicate = predicate.And(p => p.ajblGEDM > value); 
           break; 
         } 
         break; 

上面的switch語句重複了近200次，唯一不同的情況是每次都檢查字段名稱。我想盡可能地減少這些代碼。

Answer 1

您可以動態地構建表達這樣的：

string propertyName = GetPropertyName(f); 
ExpressionType comp = GetComparisonType(f); 
ParameterExpression p = Expression.Parameter(typeof(OADataConsolidated)); 
Expression<Func<OADataConsolidated, bool>> expr = 
    Expression.Lambda<Func<OADataConsolidated, bool>>(
     Expression.MakeBinary(
      comp, 
      Expression.Property(p, propertyName), 
      Expression.Constant((double)value)), 
     p); 

predicate = predicate.And(expr); 


... 

static string GetPropertyName(DOTFilter filter) 
{ 
    switch(filter.IdValue) 
    { 
     case 4: // GED: Reasoning 
      propertyName = "ajblGEDR_Mean"; 
      break; 
     case 5: // GED: Mathematics 
      propertyName = "ajblGEDM"; 
      break; 
     ... 
     default: 
      throw new ArgumentException("Unknown Id value"); 
    } 
} 

static ExpressionType GetComparisonType(DOTFilter filter) 
{ 
    switch (filter.Comp) 
    { 
     case DOTFilter.Comparitor.LessThan: 
      return ExpressionType.LessThan; 
     case DOTFilter.Comparitor.EqualOrLess: 
      return ExpressionType.LessThanOrEqual; 
     case DOTFilter.Comparitor.EqualTo: 
      return ExpressionType.Equal; 
     case DOTFilter.Comparitor.EqualOrGreater: 
      return ExpressionType.GreaterThanOrEqual; 
     case DOTFilter.Comparitor.GreaterThan: 
      return ExpressionType.GreaterThan; 
     default: 
      throw new ArgumentException("Unknown Comp value"); 
    } 
} 

開關仍然存在，但他們沒有重複。