如何使用Expression.Lambda選擇多個列？

我有一個 efcore 擴展，你可以在下面找到。它允許我選擇使用屬性路徑作為字串：

_dbContext.MyModel.Select("Name")    

代替

_dbContext.MyModel.Select(x => x.Name)

此選擇回傳IQueryable<string>. 另外我想回傳模型的 ID。如果沒有擴展，它看起來像這樣：

_dbContext.MyModel.Select(x => new { Id = x.Id, Name = x.Name });

我希望能夠打電話，例如：

_dbContext.MyModel.Select("Name", withId: true)

并回傳{ Id = x.Id, Name = x.Name }。Id 的源型別可以是字串或 Guid。目標型別應該是字串。知道如何在下面的實作中實作這一目標嗎？擴展也可以回傳 { string?, string }, string? 是身份。

Efcore 擴展：

public static IQueryable<string> SelectFromPropertyPath<T>(this IQueryable<T> query, string propertyPath)
{
    var parameter = Expression.Parameter(typeof(T), "e");
    var property = MakePropPath(parameter, propertyPath);

    if (property.Type != typeof(string))
    {
        if (property.Type != typeof(object))
        {
            property = Expression.Convert(property, typeof(object));
        }

        property = Expression.Call(_toStringMethod, property);
    }

    var lambda = Expression.Lambda<Func<T, string>>(property, parameter);

    return query.Select(lambda);
}

private static Expression MakePropPath(Expression objExpression, string path)
    => path.Split('.').Aggregate(objExpression, Expression.PropertyOrField);


private static readonly MethodInfo _toStringMethod = typeof(Convert).GetMethods()
    .Single(m =>
        m.Name == nameof(Convert.ToString) && m.GetParameters().Length == 1 &&
        m.GetParameters()[0].ParameterType == typeof(object)
    );

uj5u.com熱心網友回復：

以下擴展選擇Value并Id基于屬性名稱和回傳IQueryable<ValueWithId>，稍后可用于過濾、分組、實作等。

var query1 = _dbContext.MyModel.Select("Name"); 
var query2 = _dbContext.MyModel.Select("Name", "OtherId");

和實作：

public static class QueryableExtensions
{
    public class ValueWithId
    {
        public string Id { get; set; }
        public string Value { get; set; }
    }

    public static IQueryable<ValueWithId> Select<T>(this IQueryable<T> query, string valuePath, string idPath = "Id")
    {
        var parameter = Expression.Parameter(typeof(T), "e");
        var idProperty = MakePropPath(parameter, idPath);
        var valueProperty = MakePropPath(parameter, valuePath);

        idProperty = EnsureString(idProperty);
        valueProperty = EnsureString(valueProperty);

        var body = Expression.MemberInit(Expression.New(_valueWithIdConstructor), 
            Expression.Bind(_idProp, idProperty), 
            Expression.Bind(_valueProp, valueProperty));

        var lambda = Expression.Lambda<Func<T, ValueWithId>>(body, parameter);

        return query.Select(lambda);
    }

    static Expression MakePropPath(Expression objExpression, string path)
    {
        return path.Split('.').Aggregate(objExpression, Expression.PropertyOrField);
    }

    private static Expression EnsureString(Expression expression)
    {
        if (expression.Type == typeof(string))
            return expression;

        if (expression.Type != typeof(object))
            expression = Expression.Convert(expression, typeof(object));

        expression = Expression.Call(_toStringMethod, expression);

        return expression;
    }

    private static MethodInfo _toStringMethod = typeof(Convert).GetMethods()
        .Single(m =>
            m.Name == nameof(Convert.ToString) && m.GetParameters().Length == 1 &&
            m.GetParameters()[0].ParameterType == typeof(object)
        );

    private static readonly ConstructorInfo _valueWithIdConstructor = typeof(ValueWithId).GetConstructors().Single();
    private static readonly PropertyInfo _idProp = typeof(ValueWithId).GetProperty(nameof(ValueWithId.Id)) ?? throw new InvalidOperationException();
    private static readonly PropertyInfo _valueProp = typeof(ValueWithId).GetProperty(nameof(ValueWithId.Value)) ?? throw new InvalidOperationException();
}

標籤：C＃ 林克 实体框架核心

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