linq - 将 Expression> 发送到 Where() 扩展方法时，Ravendb Session.Query() 方法不返回任何结果

Question

我遇到了 Ravendb session.Query.Where() 的问题。当我像这样使用它时，它可以完美运行并返回所有用户名为“John”的 UserIdentitiy:

users = session.Query<UserIdentity>().Where(x => x.UserName == "John").ToList();

但是当我尝试将表达式作为变量发送到 Where 方法时，它会静默失败并且不返回任何内容。

Expression<Func<UserIdentity, bool>> whereClause = x => x.UserName == "John";

using (var session = _store.OpenSession())
{
    users = session.Query<UserIdentity>().Where(whereClause).ToList();
}

怎么了？

Answer 1

试试下面的代码:

public static class ExpressionTreesExtension
{

    public static Expression<Func<T, bool>> EqualExpression<T>(string columnName,object value)
    {
        ParameterExpression parameterType = Expression.Parameter(typeof(T), "object");

        MemberExpression typeColumn = Expression.Property(parameterType, columnName);

        ConstantExpression constant = Expression.Constant(value, typeof(string));

        BinaryExpression binaryExpression = Expression.Equal(typeColumn, constant);

        return Expression.Lambda<Func<T, bool>>(binaryExpression, parameterType);
    }
}

Code Usage:

Expression<Func<UserIdentity, bool>> whereClause = ExpressionTreesExtension.EqualExpression<UserIdentity>("UserName","John");