C#表達式樹Expression動態(tài)創(chuàng)建表達式

更新時間：2021年12月25日 11:54:23 作者：阿波Plus

這篇文章介紹了C#表達式樹Expression動態(tài)創(chuàng)建表達式的方法，對大家的學習或者工作具有一定的參考學習價值，需要的朋友們下面隨著小編來一起學習學習吧

在一些管理后臺中，對數(shù)據(jù)進行多條件查詢是一件很普遍的事情，比如在用戶列表需要實現(xiàn)可以對 "用戶名"、"手機號"、"賬戶是否凍結(jié)" 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(dǎo)致查詢接口實現(xiàn)起來堆疊了一堆看起來有用但實際很繁瑣的代碼。所以根據(jù)前后端請求報文協(xié)商，我們就可以按照一定的格式來動態(tài)創(chuàng)建表達式樹。

創(chuàng)建 QueryEntity 類

QueryEntity 是前端向 API 傳遞的參數(shù)列表，通過這個類，服務(wù)端可以知道前端需要查詢哪個字段，使用什么方法(Equals、Contains)過濾。

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

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

    /// <summary>
    /// 操作方法，對應(yīng)OperatorEnum枚舉類
    /// </summary>
    public string Operator { get; set; }

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

創(chuàng)建 OperatorEnum 類

OperatorEnum 這是一個操作方法的枚舉類，規(guī)定了 API 允許的查詢方法，比如 Equals、Contains 等等。

/// <summary>
/// 操作方法枚舉
/// </summary>
public enum OperatorEnum
{
    /// <summary>
    /// 等于
    /// </summary>
    Equals,

    /// <summary>
    /// 不等于
    /// </summary>
    NotEqual,

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

    /// <summary>
    /// 由什么開始
    /// </summary>
    StartsWith,

    /// <summary>
    /// 由什么結(jié)束
    /// </summary>
    EndsWith,

    /// <summary>
    /// 大于
    /// </summary>
    Greater,

    /// <summary>
    /// 大于等于
    /// </summary>
    GreaterEqual,

    /// <summary>
    /// 小于
    /// </summary>
    Less,

    /// <summary>
    /// 小于等于
    /// </summary>
    LessEqual,
}

創(chuàng)建 ExpressionExtension 類

ExpressionExtension 類實現(xiàn)了表達式樹的動態(tài)創(chuàng)建，將前端傳入的多條件查詢轉(zhuǎn)換成表達式，用于 EFCore 的查詢。

/// <summary>
/// 表達式擴展
/// </summary>
/// <typeparam name="T">泛型</typeparam>
public static class ExpressionExtension<T> where T : class, new()
{
    /// <summary>
    /// 表達式動態(tài)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;
            // 邏輯運算符判斷
            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>
    /// 創(chuàng)建 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'.'，說明是父表的字段
        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>
    /// 屬性類型轉(zhuǎn)換
    /// </summary>
    /// <param name="entity">查詢實體</param>
    /// <returns></returns>
    private static object ParseType(QueryEntity entity)
    {
            try
            {
                PropertyInfo property;
                // 包含'.'，說明是子類的字段
                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("字段類型轉(zhuǎn)換失?。鹤侄蚊e誤或值類型不正確");
            }
    }

    /// <summary>
    /// 創(chuàng)建 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)),
        };
    }
}

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

查詢用戶表中名稱(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")

查詢用戶表中年齡(age) 大于等于 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

查詢用戶表中名稱(name) 包含 "chen" 并且年齡(age) 大于等于 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：這里得填入 "AND",代表兩個條件是并且的關(guān)系，如果需要查詢名稱包含 "chen" 或者 年齡大于等于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

查詢用戶表中名稱(name) 包含 "chen" 并且地址(address)在廣東?。?/p>

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：這里得填入 "AND",代表兩個條件是并且的關(guān)系，如果需要查詢名稱包含 "chen" 或者 年齡大于等于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))}

到此這篇關(guān)于C#表達式樹Expression動態(tài)創(chuàng)建表達式的文章就介紹到這了。希望對大家的學習有所幫助，也希望大家多多支持腳本之家。

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