类中有个地方，老出错，帮我看看

[nbjed]

using System;
using System.Collections;
using System.Reflection;
using System.Runtime.InteropServices;
using System.Runtime.Serialization;

namespace Jedsoft.Common
{
///<Summary>
///Thread safe dictionary template
///</Summary>
[SerializableAttribute()]
// [DefaultMemberAttribute("Item")]
public class SafeDictionary<KeyType, ValueType> : Hashtable, ISerializable
{

#region Fields
///<Summary>
///If true, the dictionary allows null values
///</Summary>
protected bool mAllowNulls;
///<Summary>
///Default value
///</Summary>
protected ValueType mDefaultValue;
#endregion

#region Constructors

///<Summary>
///SafeDictionary constructor
///</Summary>
public SafeDictionary()
{
}


///<Summary>
///Deserialization constructor
///</Summary>
///<Param>
///Streaming context
///</Param>
///<Param>
///Serialization info
///</Param>
public SafeDictionary(SerializationInfo info, StreamingContext context)
: base(info, context)
{
}

#endregion

#region Properties

///<Summary>
///If true, the dictionary allows null values as valid
///</Summary>
public bool AllowNulls
{
get
{
return this.mAllowNulls;
}
set
{
this.mAllowNulls = value;
}
}


public ValueType this[KeyType key]
{
get
{
return (ValueType)base[key];
}
set
{
if ((value == null) && this.mAllowNulls)
{
value = ((ValueType)DBNull.Value);//这里出错：无法将类型“System.DBNull”转换为“ValueType”
}
base[key] = value;
}
}

#endregion

#region Methods

void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)
{
base.GetObjectData(info, context);
}

public bool TryGetValue(KeyType key, out ValueType value)
{
object object1 = base[key];
if ((object1 == DBNull.Value) && this.mAllowNulls)
{
value = default(ValueType);
return true;
}
else if (object1 != null)
{
value = ((ValueType)object1);
return true;
}
else
{
value = this.mDefaultValue;
return false;
}
}

#endregion
}
}

高手帮我解决一下

[wuyq11]

Object.Equals(obj, System.DBNull.Value)

[HsWong]

错误信息? 看提示解决嘛

[nbjed]

value = ((ValueType)DBNull.Value);//这里出错：无法将类型“System.DBNull”转换为“ValueType”
自己解决不了啊

[chouto]

ValueType是什么类型？

ValueType必须是可空的值类型才能像你写的那样转换。
比如: int? i=(int?)null;

[nbjed]

ISerializable

[wsg13793257207]

没用过该类型，回家学习

[xsm545]

友情帮顶.解决不了..

[youyou0204]

没见过这样的问题，也没用过这个类型，学习一下