表達如何寫string.Contains（someText）樹

Question

這是the tutorial我下面來詳細表達式樹。

我已經超過35列顯示，但用戶可以選擇在一次顯示10列。因此，用戶在搜索框中鍵入內容，我只想搜索用戶可見的列。

SELECT FirstName, LastName, Address, ..., State 
FROM Students 
WHERE Id == @Id col1 AND (
     FirstName LIKE '%@searchText%' OR 
     LastName LIKE '%@searchText%' OR 
     Address LIKE '%@searchText%' OR 
     ... 
     State LIKE '%@searchText%') 

返回到LINQ，這就是我正在努力完成它：

​​

這種私有方法：

private Expression<Func<Student, bool>> GetPredicate(int id, List<string> listOfColumns) 
{ 
    ParameterExpression pe = Expression.Parameter(typeof(Student), "s"); 

    Expression left0 = Expression.Property(pe, "Id"); 
    Expression right0 = Expression.Constant(id); 
    Expression e0 = Expression.Equal(left0, right0); 

    //Here ... omitted code because it's not working... 
    // 

    var expr = Expression.Lambda<Func<Student, bool>>(e0, new ParameterExpression[] { pe }); 
     return expr; 
} 

因爲它上面，它的工作就好了。但是，我甚至寫了這個方法的原因是隻能通過用戶選擇的列進行過濾。

我希望能夠基於是在UI中可見柱組成。

if(!string.IsNullOrEmpty(searchText)) 
{ 
    foreach (string columnName in columnList) 
    { 
     Expression col = Expression.Property(pe, columnName); 
     Expression left = Expression.Call(pe, typeof(string).GetMethod("Contains")); 
     Expression right = Expression.Constant(searchText); 
     Expression e = Expression.IsTrue(left, right); 
    } 
} 

我完全失去了。我知道我需要訪問字符串類的Contains方法，然後我不知道接下來會發生什麼。這樣做是爲了得到這樣的：

Where(d => d.Id == id && (d.FirstName.Contains(searchText) 
     || d.LastName.Contains(searchText) 
     || ... 
     || d.State.Contains(searchText))) 

感謝您的幫助

Answer 1

你是相當接近，除了構建的Contains通話不具有右側：

Expression col = Expression.Property(pe, columnName); 
Expression contains = Expression.Call(
    pe 
, typeof(string).GetMethod("Contains") // Make a static field out of this 
, Expression.Constant(searchText)  // Prepare a shared object before the loop 
); 

一旦你有你的電話表達，與OrElse將它們結合起來，產生的拉姆達的身體。你可以用循環做到這一點，或者你可以使用LINQ：

private static readonly MethodInfo Contains = typeof(string).GetMethod(nameof(string.Contains)); 

public static Expression<Func<Student,bool>> SearchPredicate(IEnumerable<string> properties, string searchText) { 
    var param = Expression.Parameter(typeof(Student)); 
    var search = Expression.Constant(searchText); 
    var components = properties 
     .Select(propName => Expression.Call(Expression.Property(param, propName), Contains, search)) 
     .Cast<Expression>() 
     .ToList(); 
    // This is the part that you were missing 
    var body = components 
     .Skip(1) 
     .Aggregate(components[0], Expression.OrElse); 
    return Expression.Lambda<Func<Student, bool>>(body, param); 
} 

Answer 2

我喜歡這樣的場景在PredicateBuilder類的東西：

using System; 
using System.Linq; 
using System.Linq.Expressions; 
using System.Collections.Generic; 

public static class PredicateBuilder 
{ 
    public static Expression<Func<T, bool>> True<T>() { return f => true; } 
    public static Expression<Func<T, bool>> False<T>() { return f => false; } 

    public static Expression<Func<T, bool>> Or<T> (this Expression<Func<T, bool>> expr1, 
                 Expression<Func<T, bool>> expr2) 
    { 
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression>()); 
    return Expression.Lambda<Func<T, bool>> 
      (Expression.OrElse (expr1.Body, invokedExpr), expr1.Parameters); 
    } 

    public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1, 
                 Expression<Func<T, bool>> expr2) 
    { 
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression>()); 
    return Expression.Lambda<Func<T, bool>> 
      (Expression.AndAlso (expr1.Body, invokedExpr), expr1.Parameters); 
    } 
} 

使用這種看起來像代碼：

var predicate = PredicateBuilder.True<Student>().And(i=>i.Id==id); 
if(!string.IsNullOrEmpty(searchText)) 
{ 
    if (firstNameColumnVisible) { 
     predicate = predicate.And(i=>i.FirstName.Contains(searchText)); 
    } 
    if (lastNameColumnVisible) { 
     predicate = predicate.And(i=>i.LastName.Contains(searchText)); 
    } 
    // more columns here. 
} 

最後，使用PredicateBuilder實例作爲參數傳遞給你的Linq查詢中的運算符。