使用表达式获取POCO对象的属性的名称和值 [英] Get Name and Value of a property of a POCO object using an expression

问题描述

我正在寻找一种方式来获取POCO对象中的发音的名称和价值。我尝试了许多解决方案，但似乎无法让他们工作。我真的很喜欢这个旧的解决方案，但它会导致空参考错误。

这是我想要做的事情：

  public class POCO 
 {
 public int ID {get;组; } 
 public string Name {get;组; } 
 public string Surname {get;组; } 
 public string描述{get;组; } 
} 
 
 public class POCOValidationResult 
 {
 public bool IsValid {get;组; } 
 public string Error {get;组; } 
} 
 
 public abstract class Validator< T>其中T：class 
 {
 public T Entity {get;组; } 
 
 public abstract POCOValidationResult Validate（）; 
 
 protected POCOValidationResult ValidateStringPropertyToLengthOf（Expression< Func< T，object>> expression，int maxLength）
 {
 var propertyName = getPropertyName（expression）; 
 var propertyValue = getPropertyValue（expression）; 
 
 if（propertyValue.Length> maxLength）
 {
返回新的POCOValidationResult（）
 {
错误= string.Format（{0}值太长，必须小于或等于{1}，propertyName，maxLength.ToString（））
}; 
} 
 
返回新的POCOValidationResult（）{IsValid = true}; 
} 
 
内部字符串getPropertyName（表达式< Func< T，对象>>表达式）
 {
 var memberExpersion =（MemberExpression）expression.Body; 
 
 return memberExpersion.Member.Name; 
} 
内部字符串getPropertyValue< R>（表达式< Func< T，R>>表达式）
 {
 //努力使此工作
 
 var me =（MemberExpression）expression.Body; //（MemberExpression）（（MemberExpression）expression.Body）.Expression; 
 var ce =（ConstantExpression）me.Expression; //错误在这里
 var fieldInfo = ce.Value.GetType（）。GetField（me.Member.Name，BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic）; 
 var value = fieldInfo.GetValue（ce.Value）; 
} 
} 
 
 public class POCOValidator：Validator< POCO> 
 {
 public override POCOValidationResult Validate（）
 {
 var surnameValidationResult = ValidateStringPropertyToLengthOf（p => p.Surname，10）; 
 
 if（！surnameValidationResult.IsValid）
 return surnameValidationResult; 
 
 // var descriptionValidationResult = ValidateStringPropertyToLengthOf（p => p.Description，100）; 
 
 // if（！descriptionValidationResult.IsValid）
 // return descriptionValidationResult; 
 
 // var nameValidationResult = ValidateStringPropertyToLengthOf（p => p.Name，15）; 
 
 // if（！nameValidationResult.IsValid）
 // return nameValidationResult; 
 
返回新的POCOValidationResult（）{IsValid = true}; 
} 
} 
 
 public class WorkerBee 
 {
 public void ImDoingWorkReally（）
 {
 var pocoVallidation = new POCOValidator ）
 {
 Entity = new POCO（）
 {
 ID = 1，
 Name =James，
 Surname =Dean，
描述=我不是007！ 
} 
}; 
 
 var vallidationResult = pocoVallidation.Validate（）; 
 
 if（！vallidationResult.IsValid）
 {
 return; 
} 
 
 //继续做工作... 
} 
} 
 
课程
 {
 static void Main（string [] args）
 {
 var workerBee = new WorkerBee（）; 
 
 workerBee.ImDoingWorkReally（）; 
} 
} 
  

所以你可以看到，我试图获取属性的名称和值[通过使用表达式（p => p.Surname）作为参数在方法 ValidateStringPropertyToLengthOf（... ）]。问题是，当调用 var时，我在 getPropertyValue（Expression< Func< T，object>> expression）中得到一个空参考错误ce =（ConstantExpression）me.Expression;

有没有人有想法如何让这个工作？

$ b感谢你花时间研究一下这个。我真的很感激，希望我的问题也对他人有帮助，因为我认为如果我能让这个工作起来，这可能会比较有用。

编辑：我已经在注释中进行了如下所述的更改，并且仍然收到错误无法将类型为System.Linq.Expressions.TypedParameterExpression的对象类型为System.Linq.Expressions.ConstantExpression，当我运行我的单元测试。

解决方案

我制定了一个解决方案（不幸的是，评论没有帮助）。以下是代码：

  public class POCO 
 {
 public int ID {get;组; } 
 public string Name {get;组; } 
 public string Surname {get;组; } 
 public string描述{get;组; } 
} 
 
 public class POCOValidationResult 
 {
 public bool IsValid {get;组; } 
 public string Error {get;组; } 
} 
 
 public abstract class Validator< T>其中T：class 
 {
 public T Entity {get;组; } 
 
 public abstract POCOValidationResult Validate（）; 
 
 protected POCOValidationResult ValidateStringPropertyToLengthOf（Expression< Func< T，object>> expression，int maxLength）
 {
 var propertyName = getPropertyName（expression）; 
 var propertyValue = getPropertyValue（expression）; 
 
 if（propertyValue.Length> maxLength）
 {
返回新的POCOValidationResult（）
 {
错误= string.Format（{0}值太长，必须小于或等于{1}，propertyName，maxLength.ToString（））
}; 
} 
 
返回新的POCOValidationResult（）{IsValid = true}; 
} 
 
内部字符串getPropertyName（表达式< Func< T，对象>>表达式）
 {
 var memberExpersion =（MemberExpression）expression.Body; 
 
 return memberExpersion.Member.Name; 
} 
内部字符串getPropertyValue（表达式< Func< T，对象>>表达式）
 {
 var memberExpression = expression.Body as MemberExpression; 
 var propertyInfo = memberExpression.Member as PropertyInfo; 
 
 return propertyInfo.GetValue（Entity，null）.ToString（）; 
} 
} 
 
 public class POCOValidator：Validator< POCO> 
 {
 public override POCOValidationResult Validate（）
 {
 var surnameValidationResult = ValidateStringPropertyToLengthOf（p => p.Surname，10）; 
 
 if（！surnameValidationResult.IsValid）
 return surnameValidationResult; 
 
 var descriptionValidationResult = ValidateStringPropertyToLengthOf（p => p.Description，100）; 
 
 if（！descriptionValidationResult.IsValid）
 return descriptionValidationResult; 
 
 var nameValidationResult = ValidateStringPropertyToLengthOf（p => p.Name，15）; 
 
 if（！nameValidationResult.IsValid）
 return nameValidationResult; 
 
返回新的POCOValidationResult（）{IsValid = true}; 
} 
} 
 
 public class WorkerBee 
 {
 public void ImDoingWorkReally（）
 {
 var pocoVallidation = new POCOValidator ）
 {
 Entity = new POCO（）
 {
 ID = 1，
 Name =James，
 Surname =Dean，
描述=我不是007！ 
} 
}; 
 
 var vallidationResult = pocoVallidation.Validate（）; 
 
 if（！vallidationResult.IsValid）
 {
 return; 
} 
 
 //继续做工作... 
} 
} 
 
课程
 {
 static void Main（string [] args）
 {
 var workerBee = new WorkerBee（）; 
 
 workerBee.ImDoingWorkReally（）; 
} 
} 
  

请注意 getPropertyValue的更改：

 内部字符串getPropertyValue（表达式< Func< T，对象>>表达式）
 {
 var memberExpression = expression.Body as MemberExpression; 
 var propertyInfo = memberExpression.Member as PropertyInfo; 
 
 return propertyInfo.GetValue（Entity，null）.ToString（）; 
} 
  

I'm looking for a way to get the name and value of a proptery in a POCO object. I've tried many solutions but can't seem to get them to work. I really liked this older solution but it causes a null ref error.

Here's kind of what I'm trying to do:

public class POCO
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public string Surname { get; set; }
        public string Description { get; set; }
    }

public class POCOValidationResult
{
    public bool IsValid { get; set; }
    public string Error { get; set; }
}

public abstract class Validator<T> where T : class
{
    public T Entity { get; set; }

    public abstract POCOValidationResult Validate();

    protected POCOValidationResult ValidateStringPropertyToLengthOf(Expression<Func<T, object>> expression, int maxLength)
    {
        var propertyName = getPropertyName(expression);
        var propertyValue = getPropertyValue(expression);

        if (propertyValue.Length > maxLength)
        {
            return new POCOValidationResult()
            {
                Error = string.Format("{0} value is too long. Must be less or equal to {1}", propertyName, maxLength.ToString())
            };
        }

        return new POCOValidationResult() { IsValid = true };
    }

    internal string getPropertyName(Expression<Func<T, object>> expression)
    {
        var memberExpersion = (MemberExpression)expression.Body;

        return memberExpersion.Member.Name;
    }
    internal string getPropertyValue<R>(Expression<Func<T, R>> expression)
    {
        //struggling to get this to work

        var me = (MemberExpression)expression.Body; // (MemberExpression)((MemberExpression)expression.Body).Expression;
        var ce = (ConstantExpression)me.Expression; // Error here!
        var fieldInfo = ce.Value.GetType().GetField(me.Member.Name, BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
        var value = fieldInfo.GetValue(ce.Value);
    }
}

public class POCOValidator : Validator<POCO>
{
    public override POCOValidationResult Validate()
    {
        var surnameValidationResult = ValidateStringPropertyToLengthOf(p => p.Surname, 10);

        if (!surnameValidationResult.IsValid)
            return surnameValidationResult;

        //var descriptionValidationResult = ValidateStringPropertyToLengthOf(p => p.Description, 100);

        //if (!descriptionValidationResult.IsValid)
        //    return descriptionValidationResult;

        //var nameValidationResult = ValidateStringPropertyToLengthOf(p => p.Name, 15);

        //if (!nameValidationResult.IsValid)
        //    return nameValidationResult;

        return new POCOValidationResult() { IsValid = true };
    }
}

public class WorkerBee
{
    public void ImDoingWorkReally()
    {
        var pocoVallidation = new POCOValidator()
        {
            Entity = new POCO()
            { 
                ID = 1, 
                Name = "James", 
                Surname = "Dean", 
                Description = "I'm not 007!"
            }
        };

        var vallidationResult = pocoVallidation.Validate();

        if (!vallidationResult.IsValid)
        {
            return;
        }

        //continue to do work...
    }
}

class Program
{
    static void Main(string[] args)
    {
        var workerBee = new WorkerBee();

        workerBee.ImDoingWorkReally();
    }
}

So as you can see, I'm trying to get the Property's name and value [by using an Expression (p => p.Surname) as a parameter in the method ValidateStringPropertyToLengthOf(...)]. The problem is that I'm getting a null ref error in getPropertyValue(Expression<Func<T, object>> expression) when it calls var ce = (ConstantExpression)me.Expression;

So does anyone have ideas on how to get this to work?

Thanks for taking the time to look into this. I really appreciate it and hope that my question also is helpful for others as I think this can be rather useful if I can get this to work.

EDIT: I've made the change as mentioned below in the comments and still getting the error "Unable to cast object of type 'System.Linq.Expressions.TypedParameterExpression' to type 'System.Linq.Expressions.ConstantExpression" when I run my unit test.

解决方案

I worked out a solution (unfortunately the comments were not helpful). Here's the code that works:

public class POCO
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string Surname { get; set; }
    public string Description { get; set; }
}

public class POCOValidationResult
{
    public bool IsValid { get; set; }
    public string Error { get; set; }
}

public abstract class Validator<T> where T : class
{
    public T Entity { get; set; }

    public abstract POCOValidationResult Validate();

    protected POCOValidationResult ValidateStringPropertyToLengthOf(Expression<Func<T, object>> expression, int maxLength)
    {
        var propertyName = getPropertyName(expression);
        var propertyValue = getPropertyValue(expression);

        if (propertyValue.Length > maxLength)
        {
            return new POCOValidationResult()
            {
                Error = string.Format("{0} value is too long. Must be less or equal to {1}", propertyName, maxLength.ToString())
            };
        }

        return new POCOValidationResult() { IsValid = true };
    }

    internal string getPropertyName(Expression<Func<T, object>> expression)
    {
        var memberExpersion = (MemberExpression)expression.Body;

        return memberExpersion.Member.Name;
    }
    internal string getPropertyValue(Expression<Func<T, object>> expression)
    {
        var memberExpression = expression.Body as MemberExpression;
        var propertyInfo = memberExpression.Member as PropertyInfo;

        return propertyInfo.GetValue(Entity, null).ToString();
    }
}

public class POCOValidator : Validator<POCO>
{
    public override POCOValidationResult Validate()
    {
        var surnameValidationResult = ValidateStringPropertyToLengthOf(p => p.Surname, 10);

        if (!surnameValidationResult.IsValid)
            return surnameValidationResult;

        var descriptionValidationResult = ValidateStringPropertyToLengthOf(p => p.Description, 100);

        if (!descriptionValidationResult.IsValid)
            return descriptionValidationResult;

        var nameValidationResult = ValidateStringPropertyToLengthOf(p => p.Name, 15);

        if (!nameValidationResult.IsValid)
            return nameValidationResult;

        return new POCOValidationResult() { IsValid = true };
    }
}

public class WorkerBee
{
    public void ImDoingWorkReally()
    {
        var pocoVallidation = new POCOValidator()
        {
            Entity = new POCO()
            {
                ID = 1,
                Name = "James",
                Surname = "Dean",
                Description = "I'm not 007!"
            }
        };

        var vallidationResult = pocoVallidation.Validate();

        if (!vallidationResult.IsValid)
        {
            return;
        }

        //continue to do work...
    }
}

class Program
{
    static void Main(string[] args)
    {
        var workerBee = new WorkerBee();

        workerBee.ImDoingWorkReally();
    }
}

Note the change for getPropertyValue:

    internal string getPropertyValue(Expression<Func<T, object>> expression)
    {
        var memberExpression = expression.Body as MemberExpression;
        var propertyInfo = memberExpression.Member as PropertyInfo;

        return propertyInfo.GetValue(Entity, null).ToString();
    }

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