在运行时创建动态 LINQ 查询表达式，该表达式转换为参数化 SQL 查询

Question

我想为 IQueryable 创建我的自定义表达式。示例扩展方法：

    public static IQueryable<T> GetByIntTest<T>(this IQueryable<T> qSource, Expression<Func<T, int?>> field, int? value) 
    {
        if (value == null)
            return qSource;

        ConstantExpression constantExpression = Expression.Constant(value, typeof(int?));
        BinaryExpression binaryExpression = Expression.Equal(field.Body, constantExpression);
        var predicate = Expression.Lambda<Func<T, bool>>(binaryExpression, field.Parameters);
        return qSource.Where(predicate);
    }

它可以工作，但问题是它转换为未参数化的 sql。

例如没有扩展名的代码

int userId = 3;
var testUsual = Context.Set<User>().Where(u => u.Id == userId);

翻译成下一条sql

SELECT [Extent1].[Id] AS [Id],
       [Extent1].[FirstName] AS [FirstName], 
       [Extent1].[LastName] AS [LastName], 
FROM [dbo].[User] AS [Extent1]
WHERE [Extent1].[Id] = @p__linq__0

及扩展方法

int userId = 3;
var testExtension = Context.Set<User>().GetByIntTest(u => u.Id, userId);

翻译成

SELECT [Extent1].[Id] AS [Id],
       [Extent1].[FirstName] AS [FirstName], 
       [Extent1].[LastName] AS [LastName], 
FROM [dbo].[User] AS [Extent1]
WHERE 3 = [Extent1].[Id]

那么我该如何编写表达式以在 sql 中生成类似 @p_linq_0 的内容？

更新

感谢usr的回答，我重写了我的扩展方法，现在它生成了我想要的@p_linq_0。

public static IQueryable<T> GetByIntTest<T>(this IQueryable<T> qSource, Expression<Func<T, int?>> field, int? value) 
{
    if (value == null)
        return qSource;

    var binaryExpression = Expression.Equal(field.Body, ExpressionClosureFactory.GetField(value));
    var predicate = Expression.Lambda<Func<T, bool>>(binaryExpression, field.Parameters);
    return qSource.Where(predicate);
}

public class ExpressionClosureFactory
{
    public static MemberExpression GetField<TValue>(TValue value)
    {
        var closure = new ExpressionClosureField<TValue>
        {
            ValueProperty = value
        };

        return Expression.Field(Expression.Constant(closure), "ValueProperty");
    }

    class ExpressionClosureField<T>
    {
        public T ValueProperty;
    }
}

Answer 1

我遇到过这个问题。您正在生成 u => u.Id == 3 作为表达式，而 C# 编译器将生成：

class CompilerGeneratedClosure { public int UserId; }
var closure = new CompilerGeneratedClosure() { UserId = 3 };
u => u.Id == closure.UserId

像 C# 编译器那样做。创建一个类来保存 ID 或使用元组。将closure 注入为常量表达式。