如何傳遞一個實體屬性作爲LINQ表達式的參數？

Question

如何傳遞一個實體屬性作爲LINQ表達式的參數？

public DropdownFilter(WhatTypeHere? ABC) 
{ 
    // I want to store a selected property here 
    // ABC must be a property of TEntity 
    this.ABC = ABC; 
} 

// I want the class to encapsulate a LINQ query and just parametrize it with a property 
public override IQueryable<TEntity> Filter(IQueryable<TEntity> filteredEntityCollection, string value) 
{ 
    // ABC is a property of TEntity 
    return filteredEntityCollection.Where(this.ABC == value); 
} 

我會用這樣的：

new DropdownFilter<Invoice>(invoice => invoice.SomeProperty); 

我已經與Expression<Func<TEntity, string>>各種參數的嘗試，但沒有成功。這是抱怨

The LINQ expression node type 'Invoke' is not supported in LINQ to Entities. 

Answer 1

你必須手動建立LINQ表達式。首先，我想第二個泛型參數添加到您的類，這將指定的屬性和價值型類型，你會傳遞給過濾：

public class DropdownFilter<TEntity, TProperty> 

接下來，你應該通過屬性選擇器表達式來此過濾器類的構造函數：

private PropertyInfo propertyInfo; 

public DropdownFilter(Expression<Func<TEntity, TProperty>> propertySelector) 
{ 
    this.propertyInfo = (PropertyInfo)((MemberExpression)propertySelector.Body).Member; 
} 

而且最後，構建λ表達式通過指定屬性的給定值來過濾的可查詢：

public IQueryable<TEntity> Filter(
    IQueryable<TEntity> filteredEntityCollection, TProperty value) 
{ 
    var param = Expression.Parameter(typeof(TEntity), "p"); 

    var lambda = Expression.Lambda<Func<TEntity, bool>>(    
     Expression.Equal(
      Expression.Property(param, propertyInfo), 
      Expression.Constant(value) 
     ), param); 

    return filteredEntityCollection.Where(lambda); 
} 

用法：

var filter = new DropdownFilter<Invoice, string>(i => i.ABC); 
var result = filter(db.Invoices, "foo"); // strongly-typed parameter here 

我會添加傳遞給構造函數的屬性選擇器表達式的驗證。你應該檢查它是否是MemberExpression。而且你可以確定屬性類型只支持原始類型的屬性。