[C# Expression] 之動態建立表示式

 
上一篇中說到了 Expression 的一些概念性東西，其實也是為了這一篇做知識準備。為了實現 EFCore 的多條件、連表查詢，簡化查詢程式碼編寫，也就有了這篇文章。
 
在一些管理後臺中，對資料進行多條件查詢是一件很普遍的事情，比如在使用者列表需要實現可以對 "使用者名稱"、"手機號"、"賬戶是否凍結" 等等一系列的條件查詢，常見的處理方式就是通過一系列 if...else... 來對條件進行拼接。這會導致查詢介面實現起來堆疊了一堆看起來有用但實際很繁瑣的程式碼。所以根據前後端請求報文協商，我們就可以按照一定的格式來動態建立表示式樹。
 

00、建立 QueryEntity 類

QueryEntity 是前端向 API 傳遞的引數列表，通過這個類，服務端可以知道前端需要查詢哪個欄位，使用什麼方法(Equals、Contains)過濾。

/// <summary>
/// 查詢實體
/// </summary>
public class QueryEntity
{
    /// <summary>
    /// 欄位名稱
    /// </summary>
    public string Key { get; set; }

    /// <summary>
    /// 值
    /// </summary>
    public string Value { get; set; }

    /// <summary>
    /// 操作方法，對應OperatorEnum列舉類
    /// </summary>
    public string Operator { get; set; }

    /// <summary>
    /// 邏輯運算子，只支援AND、OR
    /// </summary>
    public string LogicalOperator { get; set; }
}

 

01、建立 OperatorEnum 類

OperatorEnum 這是一個操作方法的列舉類，規定了 API 允許的查詢方法，比如 Equals、Contains 等等。

/// <summary>
/// 操作方法列舉
/// </summary>
public enum OperatorEnum
{
    /// <summary>
    /// 等於
    /// </summary>
    Equals,

    /// <summary>
    /// 不等於
    /// </summary>
    NotEqual,

    /// <summary>
    /// 包含
    /// </summary>
    Contains,

    /// <summary>
    /// 由什麼開始
    /// </summary>
    StartsWith,

    /// <summary>
    /// 由什麼結束
    /// </summary>
    EndsWith,

    /// <summary>
    /// 大於
    /// </summary>
    Greater,

    /// <summary>
    /// 大於等於
    /// </summary>
    GreaterEqual,

    /// <summary>
    /// 小於
    /// </summary>
    Less,

    /// <summary>
    /// 小於等於
    /// </summary>
    LessEqual,
}

 

02、建立 ExpressionExtension 類

ExpressionExtension 類實現了表示式樹的動態建立，將前端傳入的多條件查詢轉換成表示式，用於 EFCore 的查詢。

/// <summary>
/// 表示式擴充套件
/// </summary>
/// <typeparam name="T">泛型</typeparam>
public static class ExpressionExtension<T> where T : class, new()
{
    /// <summary>
    /// 表示式動態拼接
    /// </summary>
    public static Expression<Func<T, bool>> ExpressionSplice(List<QueryEntity> entities)
    {
        if (entities.Count < 1)
        {
            return ex => true;
        }
        var expression_first = CreateExpressionDelegate(entities[0]);
        foreach (var entity in entities.Skip(1))
        {
            var expression = CreateExpressionDelegate(entity);
            InvocationExpression invocation = Expression.Invoke(expression_first, expression.Parameters.Cast<Expression>());
            BinaryExpression binary;
            // 邏輯運算子判斷
            if (entity.LogicalOperator.ToUpper().Equals("OR"))
            {
                binary = Expression.Or(expression.Body, invocation);
            }
            else
            {
                binary = Expression.And(expression.Body, invocation);
            }
            expression_first = Expression.Lambda<Func<T, bool>>(binary, expression.Parameters);
        }
        return expression_first;
    }

    /// <summary>
    /// 建立 Expression<TDelegate>
    /// </summary>
    private static Expression<Func<T, bool>> CreateExpressionDelegate(QueryEntity entity)
    {
        ParameterExpression param = Expression.Parameter(typeof(T));

        Expression key = param;
        var entityKey = entity.Key.Trim();
        // 包含'.'，說明是父表的欄位
        if (entityKey.Contains('.'))
        {
            var tableNameAndField = entityKey.Split('.');
            key = Expression.Property(key, tableNameAndField[0].ToString());
            key = Expression.Property(key, tableNameAndField[1].ToString());
        }
        else
        {
            key = Expression.Property(key, entityKey);
        }

        Expression value = Expression.Constant(ParseType(entity));
        Expression body = CreateExpression(key, value, entity.Operator);
        var lambda = Expression.Lambda<Func<T, bool>>(body, param);
        return lambda;
    }

    /// <summary>
    /// 屬性型別轉換
    /// </summary>
    /// <param name="entity">查詢實體</param>
    /// <returns></returns>
    private static object ParseType(QueryEntity entity)
    {
            try
            {
                PropertyInfo property;
                // 包含'.'，說明是子類的欄位
                if (entity.Key.Contains('.'))
                {
                    var tableNameAndField = entity.Key.Split('.');

                    property = typeof(T).GetProperty(tableNameAndField[0], BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
                    property = property.PropertyType.GetProperty(tableNameAndField[1], BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
                }
                else
                {
                    property = typeof(T).GetProperty(entity.Key, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
                }

                return Convert.ChangeType(entity.Value, property.PropertyType);
            }
            catch (Exception)
            {
                throw new ArgumentException("欄位型別轉換失敗：欄位名錯誤或值型別不正確");
            }
    }

    /// <summary>
    /// 建立 Expression
    /// </summary>
    private static Expression CreateExpression(Expression left, Expression value, string entityOperator)
    {
        if (!Enum.TryParse(entityOperator, true, out OperatorEnum operatorEnum))
        {
            throw new ArgumentException("操作方法不存在,請檢查operator的值");
        }

        return operatorEnum switch
        {
                OperatorEnum.Equals => Expression.Equal(left, Expression.Convert(value, left.Type)),
                OperatorEnum.NotEqual => Expression.NotEqual(left, Expression.Convert(value, left.Type)),
                OperatorEnum.Contains => Expression.Call(left, typeof(string).GetMethod("Contains", new Type[] { typeof(string) }), value),
                OperatorEnum.StartsWith => Expression.Call(left, typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) }), value),
                OperatorEnum.EndsWith => Expression.Call(left, typeof(string).GetMethod("EndsWith", new Type[] { typeof(string) }), value),
                OperatorEnum.Greater => Expression.GreaterThan(left, Expression.Convert(value, left.Type)),
                OperatorEnum.GreaterEqual => Expression.GreaterThanOrEqual(left, Expression.Convert(value, left.Type)),
                OperatorEnum.Less => Expression.LessThan(left, Expression.Convert(value, left.Type)),
                OperatorEnum.LessEqual => Expression.LessThanOrEqual(left, Expression.Convert(value, left.Type)),
                _ => Expression.Equal(left, Expression.Convert(value, left.Type)),
        };
    }
}

 

03、使用示例

例如有以下兩個實體類，Address 是 User 的子類

public class User
{
    public int Id { get; set; }

    public string Name { get; set; } = string.Empty;

    public int Age { get; set; }

    public DateTime CreateTime { get; set; }

    public Address Address { get; set; }

}

public class Address
{
    public string Province { get; set; }

    public string City { get; set; }
}

單條件查詢

查詢使用者表中名稱(name) 包含 "chen" ：

List<QueryEntity> list = new List<QueryEntity>
{
    new QueryEntity
    {
        Key = "name",
        Value = "chen",
        Operator = "Contains"
    }
};
var expression = ExpressionExtension<User>.ExpressionSplice(list);
// expression = Param_0 => Param_0.Name.Contains("chen")

查詢使用者表中年齡(age) 大於等於 18：

List<QueryEntity> list = new List<QueryEntity>
{
    new QueryEntity
    {
        Key = "age",
        Value = "18",
        Operator = "GreaterEqual"
    }
};
var expression = ExpressionExtension<User>.ExpressionSplice(list);
// expression = Param_0 => Param_0.Name.GreaterThanOrEqual(18)

多條件查詢

查詢使用者表中名稱(name) 包含 "chen" 並且年齡(age) 大於等於 18：

List<QueryEntity> list = new List<QueryEntity>
{
    new QueryEntity
    {
        Key = "name",
        Value = "chen",
        Operator = "Contains"
    },
    new QueryEntity
    {
        Key = "age",
        Value = "18",
        Operator = "GreaterEqual",
        // 注意：這裡得填入 "AND",代表兩個條件是並且的關係，如果需要查詢名稱包含 "chen" 或者 年齡大於等於18，則填入 "OR"
        "logicalOperator": "AND"
    }
};
var expression = ExpressionExtension<User>.ExpressionSplice(list);
// expression = Param_0 => ((Param_0.Status >= Convert(1, Int32)) And Invoke(Param_1 => Param_1.OpenId.Contains("9JJdFTVt6oimCgdbW61sk"), Param_0))

多表查詢

查詢使用者表中名稱(name) 包含 "chen" 並且 地址(address)在廣東省：

List<QueryEntity> list = new List<QueryEntity>
{
    new QueryEntity
    {
        Key = "name",
        Value = "chen",
        Operator = "Contains"
    },
    new QueryEntity
    {
        Key = "address.Province",
        Value = "廣東省",
        Operator = "Equals",
        // 注意：這裡得填入 "AND",代表兩個條件是並且的關係，如果需要查詢名稱包含 "chen" 或者 年齡大於等於18，則填入 "OR"
        "logicalOperator": "AND"
    }
};

var expression = ExpressionExtension<BookingRecord>.ExpressionSplice(list);
// expression = {Param_0 => ((Param_0.Address.Province == Convert("廣東省", String)) And Invoke(Param_1 => Param_1.Name.Contains("chen"), Param_0))}