如何在动态选择中使用表达式创建“内联 if 语句”以进行 null 检查

如何在动态选择中使用表达式创建“内联 if 语句”以进行 null 检查？

我为对象的嵌套属性编写了一个动态选择 LINQ 表达式，但当它为 null 时，它会引发异常。所以我想检查该属性是否为空，就这么简单！

这就是我的意思：

X.Where(...)
 .Select(X => new Y{
    ...
    Z = X.Titles == null ? "" : [Linq]
    ...
}).FirstOrDefault();

这是我写的

private static Expression GetLocalizedString(Expression stringExpression, SupportedCulture supportedCulture)
    {
        var expression = Expression.Parameter(typeof(APILocalizedString), nameof(APILocalizedString));
        
        var prop = Expression.Property(expression, nameof(APILocalizedString.SupportedCulture));
        var value = Expression.Constant(supportedCulture);
        var condition = Expression.Equal(prop, value);

        var where = Expression.Call(
            typeof (Enumerable),
            nameof(Enumerable.Where),
            new Type[] { typeof(APILocalizedString) },
            stringExpression,
            Expression.Lambda<Func<APILocalizedString, bool>>(condition, expression));

        var select = Expression.Call(
            typeof(Enumerable),
            nameof(Enumerable.Select),
            new Type[] { typeof(APILocalizedString), typeof(string) },
            where,
            Expression.Lambda<Func<APILocalizedString, string>>(
                Expression.Property(expression, nameof(APILocalizedString.Text)),
                expression
            ));

        var first = Expression.Call(
            typeof(Enumerable),
            nameof(Enumerable.First),
            new Type[] { typeof(APILocalizedString) },
            stringExpression);

        var defaultIfEmpty = Expression.Call(
            typeof(Enumerable),
            nameof(Enumerable.DefaultIfEmpty),
            new Type[] { typeof(string) },
            select,
            first);

        var firstOrDefault =
            Expression.Call(
            typeof(Enumerable),
            nameof(Enumerable.FirstOrDefault),
            new Type[] { typeof(string) },
            defaultIfEmpty);


        var nullCheck = Expression.Equal(stringExpression, Expression.Constant(null, stringExpression.Type));
        var result = Expression.IfThenElse(nullCheck, Expression.Constant(""), firstOrDefault);
            
        return result;
    }

这是 GetLocalizedString 生成的内容：

{IIF((X.Titles == null), "", X.Titles.Where(APILocalizedString => (APILocalizedString.SupportedCulture == EN)).DefaultIfEmpty(X.Titles.First()).Select(APILocalizedString => APILocalizedString.Text).FirstOrDefault())}

选择表达式

... bindings.Add(Expression.Bind(property, GetLocalizedString(Expression.Property(parameter, "Titles"), SupportedCulture.EN))); ...

这是错误消息：

System.ArgumentException: 'Argument types do not match'

Select 属性的类型为 String

有什么办法可以创建像这样的表达式X.Titles == null ? "" : [Linq] ?

相当于 C# 条件的表达式?:运算符是Expression.Condition https://learn.microsoft.com/en-us/dotnet/api/system.linq.expressions.expression.condition?view=netframework-4.7.2. While Expression.IfThenElse https://learn.microsoft.com/en-us/dotnet/api/system.linq.expressions.expression.ifthenelse?view=netframework-4.7.2你正在使用的相当于C#if then else block.

两种方法都返回ConditionalExpression https://learn.microsoft.com/en-us/dotnet/api/system.linq.expressions.conditionalexpression?view=netframework-4.7.2 with Test, IfTrue and IfFalse已填充属性。不同的是结果Type of the Condition是操作数的类型，而对于IfThenElse it is void，因此不能在查询表达式树中使用。

所以你的具体问题的答案是：

var result = Expression.Condition(nullCheck, Expression.Constant(""), firstOrDefault);

附：作为一个侧面节点，我从您的代码片段中收到了几个错误，因此我必须像这样重新排列它，以便使上面的行没有错误：

private static Expression GetLocalizedString(Expression stringExpression, SupportedCulture supportedCulture)
{
    var expression = Expression.Parameter(typeof(APILocalizedString), nameof(APILocalizedString));

    var prop = Expression.Property(expression, nameof(APILocalizedString.SupportedCulture));
    var value = Expression.Constant(supportedCulture);
    var condition = Expression.Equal(prop, value);

    var where = Expression.Call(
        typeof(Enumerable),
        nameof(Enumerable.Where),
        new Type[] { typeof(APILocalizedString) },
        stringExpression,
        Expression.Lambda<Func<APILocalizedString, bool>>(condition, expression));

    var first = Expression.Call(
        typeof(Enumerable),
        nameof(Enumerable.First),
        new Type[] { typeof(APILocalizedString) },
        stringExpression);

    var defaultIfEmpty = Expression.Call(
        typeof(Enumerable),
        nameof(Enumerable.DefaultIfEmpty),
        new Type[] { typeof(APILocalizedString) },
        where,
        first);

    var select = Expression.Call(
        typeof(Enumerable),
        nameof(Enumerable.Select),
        new Type[] { typeof(APILocalizedString), typeof(string) },
        defaultIfEmpty,
        Expression.Lambda<Func<APILocalizedString, string>>(
            Expression.Property(expression, nameof(APILocalizedString.Text)),
            expression
        ));

    var firstOrDefault =
        Expression.Call(
        typeof(Enumerable),
        nameof(Enumerable.FirstOrDefault),
        new Type[] { typeof(string) },
        select);


    var nullCheck = Expression.Equal(stringExpression, Expression.Constant(null, stringExpression.Type));
    var result = Expression.Condition(nullCheck, Expression.Constant(""), firstOrDefault);

    return result;
}