使用NVelocity实现代码生成

首先实现一个NVelocity的工具类：

Imports System.IO

Imports NVelocity
Imports NVelocity.App

Public Class Velocity

    Private Shared _Templates As New Dictionary(Of String, Template)

    Public Shared Sub RegisterTemplate(name As String, filePath As String)
        If Not _Templates.ContainsKey(name) Then
            _Templates.Add(name, Engine.GetTemplate(filePath))
        End If
    End Sub

    Private Shared _Engine As VelocityEngine = Nothing
    Public Shared ReadOnly Property Engine As VelocityEngine
        Get
            If _Engine Is Nothing Then
                _Engine = New VelocityEngine()
                _Engine.Init()
            End If
            Return _Engine
        End Get
    End Property

    Public Shared Function Merge(templateName As String, parameters As IDictionary(Of String, Object)) As String
        Dim _Context As New VelocityContext()
        For Each key As String In parameters.Keys
            _Context.Put(key, parameters.Item(key))
        Next
        Using sw As New StringWriter
            _Templates.Item(templateName).Merge(_Context, sw)
            Return sw.ToString()
        End Using
    End Function

End Class

这里的RegisterTemplate方法，通过名称/路径的方式将Velocity模板文件注册给工具类。而通过Merge方法即可根据传入的参数填写到模板中并返回结果。
参考NVelocity模板文件如下：

Imports System
Imports System.Runtime.Serialization
Imports System.Collections.Generic

Namespace ${Namespace}.Model
    
    <Serializable()> _
    <DataContract()> _
    Public Partial Class ${Table.Name}DTO                   
        Inherits ModelBase

#foreach ($Column in ${Table.Columns})
        Private _${Column.Name} As ${Column.Type}
#end
#foreach ($Column in ${Table.Columns})

        Public Property ${Column.Name} As ${Column.Type}
            Get
                Return _${Column.Name}
            End Get
            Set(value As ${Column.Type})
                _${Column.Name} = value
            End Set
        End Property
#end

    End Class
End Namespace

调用代码如下（其中Table和Column为描述表及列信息的数据实体类）：

Velocity.RegisterTemplate("dto", "dto.vm")

Dim t As New Table("AccBatch")

t.Add(New Column With {.Name = "BATCHID", .SqlType = "int", .IsNullable = "0"})
t.Add(New Column With {.Name = "DATE", .SqlType = "datetime", .IsNullable = "1"})
t.Add(New Column With {.Name = "STARTTIME", .SqlType = "datetime", .IsNullable = "1"})
t.Add(New Column With {.Name = "ENDTIME", .SqlType = "datetime", .IsNullable = "1"})
t.Add(New Column With {.Name = "SUCCESSFLAG", .SqlType = "char", .IsNullable = "1"})
t.Add(New Column With {.Name = "UPDATETIME", .SqlType = "datetime", .IsNullable = "1"})
t.Add(New Column With {.Name = "EXECUTEFLAG", .SqlType = "char", .IsNullable = "1"})

Dim parameters As New Dictionary(Of String, Object)
parameters.Add("Namespace", "cn.com.dhc.Hiway")
parameters.Add("Table", t)

Using swVb As New StreamWriter(vb)
    swVb.Write(Velocity.Merge("dto", parameters))
    swVb.Flush()
End Using

通过如上方法即可生成代码如下（代码生成结果）：

Imports System
Imports System.Runtime.Serialization
Imports System.Collections.Generic

Namespace cn.com.dhc.Hiway.Model
    
    <Serializable()> _
    <DataContract()> _
    Public Partial Class AccBatchDTO                   
        Inherits ModelBase

        Private _BATCHID As Integer
        Private _DATE As DateTime?
        Private _STARTTIME As DateTime?
        Private _ENDTIME As DateTime?
        Private _SUCCESSFLAG As String
        Private _UPDATETIME As DateTime?
        Private _EXECUTEFLAG As String

        Public Property BATCHID As Integer
            Get
                Return _BATCHID
            End Get
            Set(value As Integer)
                _BATCHID = value
            End Set
        End Property

        Public Property DATE As DateTime?
            Get
                Return _DATE
            End Get
            Set(value As DateTime?)
                _DATE = value
            End Set
        End Property

        Public Property STARTTIME As DateTime?
            Get
                Return _STARTTIME
            End Get
            Set(value As DateTime?)
                _STARTTIME = value
            End Set
        End Property

        Public Property ENDTIME As DateTime?
            Get
                Return _ENDTIME
            End Get
            Set(value As DateTime?)
                _ENDTIME = value
            End Set
        End Property

        Public Property SUCCESSFLAG As String
            Get
                Return _SUCCESSFLAG
            End Get
            Set(value As String)
                _SUCCESSFLAG = value
            End Set
        End Property

        Public Property UPDATETIME As DateTime?
            Get
                Return _UPDATETIME
            End Get
            Set(value As DateTime?)
                _UPDATETIME = value
            End Set
        End Property

        Public Property EXECUTEFLAG As String
            Get
                Return _EXECUTEFLAG
            End Get
            Set(value As String)
                _EXECUTEFLAG = value
            End Set
        End Property

    End Class
End Namespace

NVelocity是不错的模板引擎，支持变量、对象、对象方法、循环、判断等模板语法，强大的模板模型支持使得使用其进行模板开发能大大提高系统效率，值得使用。