C#表达式树Expression基础讲解

[HttpGet]
public IActionResult Expression()
{
    // 查询 年龄Age 大于 18 的元素
    Expression<Func<User,bool>> expression1 = x => x.Age > 18; 
    return Ok();
}

PM> Install-Package ZSpitz.Util -Version 0.1.116

Expression<Func<User, bool>> expression = u => u.Age >= 18;
var treeStr = expression.ToString("Object notation", "C#");

// 输出为下面字符串
var u = new ParameterExpression {
    Type = typeof(User),
    IsByRef = false,
    Name = "u"
};

new Expression<Func<User, bool>> {
    NodeType = ExpressionType.Lambda,
    Type = typeof(Func<User, bool>),
    Parameters = new ReadOnlyCollection<ParameterExpression> {
        u
    },
    Body = new BinaryExpression {
        NodeType = ExpressionType.GreaterThanOrEqual,
        Type = typeof(bool),
        Left = new MemberExpression {
            Type = typeof(int),
            Expression = u,
            Member = typeof(User).GetProperty("Age")
        },
        Right = new ConstantExpression {
            Type = typeof(int),
            Value = 18
        }
    },
    ReturnType = typeof(bool)
}

// 这种写法,实际生成的 SQL 语句, 大概是这样的 SELECT * FROM User as T WHERE T.age > 18
Expression<Func<User,bool>> expression1 = x => x.Age > 18;
dbContext.User.Where(expression1).toList();

// 而这种, 生成的语句是这样的 SELECT * FROM User, 然后将 User 表中所有数据加载到内存中后, 在进行 age > 18 的过滤
Func<User, bool> func1 = x => x.Age > 18;
dbContext.User.Where(func1).toList();

public IActionResult GetUserByManualExpression()
{
    ParameterExpression parameterExpression = Expression.Parameter(type:typeof(User), name: "u");
    ConstantExpression right = Expression.Constant(18);
    MemberExpression left = Expression.MakeMemberAccess(parameterExpression, member: typeof(User).GetProperty("Age"));
    BinaryExpression body = Expression.GreaterThanOrEqual(left, right);

    Expression<Func<User, bool>> expression = Expression.Lambda<Func<User, bool>>(body, parameters: parameterExpression);

    var data = _userService.GetUsers(expression);

    return Ok(new
    {
        code = 200,
        msg = "OK",
        data
    });
}