发生“System.InvalidCastException”类型的未处理异常...无法将“System.String”类型的对象转换为类型

Question

有人可以帮帮我吗？按照我的代码和“GelatoProject.exe 中发生‘System.InvalidCastException’类型的未处理异常

附加信息：无法将“System.String”类型的对象转换为“GelatoProject.RecipeVariables”类型。”是消息。

// ...
using (var db = new GelatoProjectDBEntities())
{
    RecipeVariables selected = 
        (RecipeVariables)comboBoxRecipeDetail.SelectedItem;

    var results = (from x in db.Recipe_Parameters
                select new RecipeVariables 
                        { 
                            Id = x.Id, 
                            RecipeDetail = x.recipeDetail, 
                            Parameter = x.parameter, 
                            RangeDetail = x.rangeDetail, 
                            RangeValue = x.value.ToString() 
                        }
               )
                .Where(x => x.RecipeDetail == selected.RecipeDetail && x.Parameter == "totalsolids" && x.RangeDetail == "Max")
                .FirstOrDefault();

    totsolidsRangeMax = decimal.Parse(results.RangeValue);

    MessageBox.Show(totsolidsRangeMax.ToString());
}
// ...

class RecipeVariables
{
    public int Id { get; set; }
    public string NameIngredient { get; set; }
    public string Brand { get; set; }
    public string LBName { get { return NameIngredient + " - " + Brand;}}
    public string RecipeDetail { get; set; }
    public string Parameter { get; set; }
    public string RangeDetail { get; set; }
    public string RangeValue { get; set; }
}

Answer 1

RecipeVariables selected = (RecipeVariables)comboBoxRecipeDetail.SelectedItem;

comboBoxRecipeDetail.SelectedItem 是一个字符串 - 当您单击组合框时看到的文本。不能转换为RecipeVariables

将您的代码更改为：

using (var db = new GelatoProjectDBEntities())
{
    RecipeVariables selected = new RecipeVariables()
                               {
                                   RecipeDetail = (string)comboBoxRecipeDetail.SelectedItem
                               };

    // var results = ...
}

这将创建一个新的 RecipeVariables 对象，然后将其 RecipeDetail 属性设置为所选组合框项的文本。