如何在VB.NET中获取datatable列的最大长度 [英] How to do get max length of datatable column in VB.NET

问题描述

Hi,

For Each dc In ds.Tables(0).Columns
            dc.ColumnMapping = MappingType.Attribute
 		writer.WriteAttributeString("dt", "type", "uuid:C2F41010-65B3-11d1-A29F-00AA00C14882", _
                  GetDatatype(dc.DataType.ToString))
            writer.WriteAttributeString("dt", "maxlength", "uuid:C2F41010-65B3-11d1-A29F-00AA00C14882", _
                  dc.MaxLength.ToString)
            writer.WriteAttributeString("rs", "maybenull", "urn:schemas-microsoft-com:rowset", _
                  dc.AllowDBNull.ToString)           
            writer.WriteEndElement()           
            writer.WriteEndElement()
            writer.Flush()            
        Next
        dc = Nothing

这是我的代码我试图获得数据类型列的数据类型和最大长度，但是 maxlength对所有列显示为-1 如何解决问题，任何人都给出建议。



我尝试过的事情：



我已经尝试了上面的代码，它给出了正确的数据类型，但MAxlength对所有列都显示为-1。

This is my code i tried to get Datatype and max length of datatable columns, its working but maxlength shows -1 for all columns how to solve the problem,any one give suggestion.

What I have tried:

I have tried the above code , its give correct datatype but MAxlength shows -1 for all columns.

推荐答案

据我所知，您无法从DataTable的DataColumn中获取底层数据库列的最大长度。文档说它被非文本列忽略： DataColumn.MaxLength属性（System.Data） [ ^ ]并且对于未指定长度的列，它返回-1。

并使用DataAdapter加载DataTable来自DB不会使基础SQL列长度超过 - 它为NVARCHAR（255）列报告-1，并允许我将列中的值设置为大于255.

我怀疑你必须直接查询底层数据源，而不是试图通过它提供的数据获取信息。

As far as a know, you can't get the max length of the underlying database column from the DataColumn of a DataTable. The documentation says that it's ignored for non-text columns: DataColumn.MaxLength Property (System.Data)[^] and that it returns -1 for columns with no length assigned.
And using a DataAdapter to load a DataTable from by DB doesn't bring the underlying SQL column length over - it both reports -1 for a NVARCHAR(255) column, and allows me to set the value in the column to greater than 255.
I suspect you will have to query the underlying data source directly instead of trying to get the info "second hand" via the data it supplied.

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