如何在Linq where子句中指定动态字段名称?

时间:2010-03-23 02:56:27

标签: c# linq where-clause

如果您创建一个Filter对象,该对象包含Linq的标准,通常在这样的where子句中输入:

 var myFilterObject = FilterFactory.GetBlank();
 myFilterObject.AddCondition("Salary", "lessThan", "40000");

 var myResult = myRepository.GetEmployees(myFilterObject);

如果不使用大案例声明,如何将Linq字段与字段名称匹配?

 return from e in db.Employee
        where e.Salary < 40000
        select new IList<EmployeeViewModel> { Name= e.name, Salary= e.Salary };

我假设您需要将一个对象发送到指定过滤的存储库,以便您只提取所需的记录。我假设Linq没有预编译(除非您创建自定义委托和函数),因此您应该能够动态指定要过滤的字段。

如果你能像某些类型的Expando对象那样做e [“Salary”],那将会很不错。

3 个答案:

答案 0 :(得分:8)

我建议您查看Linq to SQL示例中的Dynamic Query。您可以使用它以“纯文本”编写条件,例如:

var employees = db.Employee.Where("Salary < 40000").Select(...);

澄清一下:这些扩展本质上是通过lambdas构造的字符串构建相同的表达式树,因此这与在数据库中编写原始SQL不同。唯一的缺点是你不能使用查询理解语法(from x in y等)。

答案 1 :(得分:8)

您可以手动构建表达式,如下所示:

var eParam = Expression.Parameter(typeof(Employee), "e");

var comparison = Expression.Lambda(
    Expression.LessThan(
        Expression.Property(eParam, "Salary"),
        Expression.Constant(40000)),
    eParam);

return from e in db.Employee.Where(comparison)
       select new EmployeeViewModel { Name = e.name, Salary = e.Salary };

答案 2 :(得分:3)

我发现这个派对已经很晚了 - 但是我已经编写了一些代码,我相信这些代码不会使用DynamicLINQ,我发现它支持得很差,并且使用起来很狡猾(充其量)。

    /// <summary>
    ///     A method to create an expression dynamically given a generic entity, and a propertyName, operator and value.
    /// </summary>
    /// <typeparam name="TEntity">
    ///     The class to create the expression for. Most commonly an entity framework entity that is used
    ///     for a DbSet.
    /// </typeparam>
    /// <param name="propertyName">The string value of the property.</param>
    /// <param name="op">An enumeration type with all the possible operations we want to support.</param>
    /// <param name="value">A string representation of the value.</param>
    /// <param name="valueType">The underlying type of the value</param>
    /// <returns>An expression that can be used for querying data sets</returns>
    private static Expression<Func<TEntity, bool>> CreateDynamicExpression<TEntity>(string propertyName,
        Operator op, string value, Type valueType)
    {
        Type type = typeof(TEntity);
        object asType = AsType(value, valueType);
        var p = Expression.Parameter(type, "x");
        var property = Expression.Property(p, propertyName);
        MethodInfo method;
        Expression q;

        switch (op)
        {
            case Operator.Gt:
                q = Expression.GreaterThan(property, Expression.Constant(asType));
                break;
            case Operator.Lt:
                q = Expression.LessThan(property, Expression.Constant(asType));
                break;
            case Operator.Eq:
                q = Expression.Equal(property, Expression.Constant(asType));
                break;
            case Operator.Le:
                q = Expression.LessThanOrEqual(property, Expression.Constant(asType));
                break;
            case Operator.Ge:
                q = Expression.GreaterThanOrEqual(property, Expression.Constant(asType));
                break;
            case Operator.Ne:
                q = Expression.NotEqual(property, Expression.Constant(asType));
                break;
            case Operator.Contains:
                method = typeof(string).GetMethod("Contains", new[] {typeof(string)});
                q = Expression.Call(property, method ?? throw new InvalidOperationException(),
                    Expression.Constant(asType, typeof(string)));
                break;
            case Operator.StartsWith:
                method = typeof(string).GetMethod("StartsWith", new[] {typeof(string)});
                q = Expression.Call(property, method ?? throw new InvalidOperationException(),
                    Expression.Constant(asType, typeof(string)));
                break;
            case Operator.EndsWith:
                method = typeof(string).GetMethod("EndsWith", new[] {typeof(string)});
                q = Expression.Call(property, method ?? throw new InvalidOperationException(),
                    Expression.Constant(asType, typeof(string)));
                break;
            default:
                throw new ArgumentOutOfRangeException(nameof(op), op, null);
        }

        return Expression.Lambda<Func<TEntity, bool>>(q, p);
    }

    /// <summary>
    ///     Extract this string value as the passed in object type
    /// </summary>
    /// <param name="value">The value, as a string</param>
    /// <param name="type">The desired type</param>
    /// <returns>The value, as the specified type</returns>
    private static object AsType(string value, Type type)
    {
        //TODO: This method needs to be expanded to include all appropriate use cases
        string v = value;
        if (value.StartsWith("'") && value.EndsWith("'"))
            v = value.Substring(1, value.Length - 2);

        if (type == typeof(string))
            return v;
        if (type == typeof(DateTime))
            return DateTime.Parse(v);
        if (type == typeof(DateTime?))
            return DateTime.Parse(v);
        if (type == typeof(int))
            return int.Parse(v);
        if (type == typeof(int?)) return int.Parse(v);

        throw new ArgumentException("A filter was attempted for a field with value '" + value + "' and type '" +
                                    type + "' however this type is not currently supported");
    }

这段代码可以这样使用:

var whereClause = CreateDynamicExpression<MyDatabaseTable>("MyFieldName",
                    Operator.Contains, "some string value",typeof(string));
var results = _db.MyDatabaseTable.Where(whereClause);