Question

Hi, I'm trying to build an Expression to get a generic entity by its primary key and getting a parameterized sql query.

Currently I can get the correct WHERE query, but it isn't parameterized.

public async Task<TDbo> Get(TKey key, Expression<Func<TEntity, TKey>> keySelector)
{
    var propertyRef = keySelector.Body;
    var parameter = keySelector.Parameters[0];
    var constantRef = Expression.Constant(key);
    var equals = Expression.Equal(propertyRef, constantRef);
    var comparer = Expression.Lambda<Func<TEntity, bool>>(equals, parameter);

    return await _context.Set<TDbo>().SingleOrDefaultAsync(comparer);
}

This results in the following query: SELECT e.\"Id\", e.\"Name\" \r\n FROM \"People\" AS e\r\nWHERE e.\"Id\" = 1\r\nLIMIT 2, instead of the wanted: SELECT e.\"Id\", e.\"Name\" \r\n FROM \"People\" AS e\r\nWHERE e.\"Id\" = @__s_0\r\nLIMIT 2

Answer 1

这是因为Session_End。值常量表达式未由查询转换器参数化。您需要的是一个引用另一个表达式的属性或字段的表达式（可以为常数）。基本上，这就是C＃编译器发出的用于关闭的内容。

一种方法是实际使用C＃编译器创建带闭包的lambda表达式并获取主体：

Expression.Constant(key)

（{Expression<Func<TKey>> keyValue = () => key; var variableRef = key.Body;代替了您的variableRef）

另一种方法是使用匿名，元组或特定的类类型创建显式的闭包实例并绑定相应的属性或字段。例如，使用匿名类型：

constantRef

或使用var variableRef = Expression.Property(Expression.Constant(new { key }), "key");：

System.Tuple

实际方法并不重要（我个人更喜欢使用lambda的第一个变体）-所有这些都会导致EF Core查询翻译器创建参数。

Building a parameterized EntityFramework Core Expression

1 个答案: