基于另一个 LINQ 表达式和值构建特定的 LINQ 表达式

发布于 2024-11-07 02:29:43 字数 372 浏览 0 评论 0原文

如果我有以下形式的 LINQ 表达式：

Expression<Func<MyClass, string, bool>> filterExpression = (x, filterVal) => x.DisplayName.Contains(filterVal);

有什么方法可以获取下面的表达式吗？

Expression<Func<MyClass, bool>> filter = x => x.DisplayName.Contains("John");

我需要在 Linq-to-EntitiesWhere 调用中使用第二个表达式。

原文

If I've got a LINQ expression of the form:

Expression<Func<MyClass, string, bool>> filterExpression = (x, filterVal) => x.DisplayName.Contains(filterVal);

Is there any way I can get to the expression below?

Expression<Func<MyClass, bool>> filter = x => x.DisplayName.Contains("John");

I need to use the second expression in a Linq-to-Entities Where call.

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岛徒 2024-11-14 02:29:43

您需要编写一个 ExpressionVisitor 用 ConstantExpression 替换 ParameterExpression。

它看起来像

protected override Expression VisitParameter(ParameterExpression node) {
    if (node.Name == "filterVal")
        return Expression.Constant(something);
    return node;
}

You need to write an ExpressionVisitor that replaces the ParameterExpression with a ConstantExpression.

It would look something like

protected override Expression VisitParameter(ParameterExpression node) {
    if (node.Name == "filterVal")
        return Expression.Constant(something);
    return node;
}

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爱，才寂寞 2024-11-14 02:29:43

如果我解决它的方式有用的话：

public class MyVisitor : ExpressionVisitor
{
    protected override Expression VisitParameter(ParameterExpression node)
    {
        Console.WriteLine("Visiting Parameter: " + node.Name);
        if (node.Name == "filterVal")
        {
            return Expression.Constant("John");
        }
        return node;
    }
}

Expression<Func<MyClass, string, bool>> filterExpression = (x, filterVal) => x.DisplayName.Contains(filterVal);
var filterExpBody = (new MyVisitor()).Visit(filterExpression.Body);
Expression<Func<MyClass, bool>> filter = Expression.Lambda<Func<MyClass, bool>>(filterExpBody, filterExpression.Parameters[0]);

In case it's useful the way I solved it was:

public class MyVisitor : ExpressionVisitor
{
    protected override Expression VisitParameter(ParameterExpression node)
    {
        Console.WriteLine("Visiting Parameter: " + node.Name);
        if (node.Name == "filterVal")
        {
            return Expression.Constant("John");
        }
        return node;
    }
}

Expression<Func<MyClass, string, bool>> filterExpression = (x, filterVal) => x.DisplayName.Contains(filterVal);
var filterExpBody = (new MyVisitor()).Visit(filterExpression.Body);
Expression<Func<MyClass, bool>> filter = Expression.Lambda<Func<MyClass, bool>>(filterExpBody, filterExpression.Parameters[0]);

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