How to initialize various type variables in dynamically generated C# code?

Question

I have T4 template where I'd like to generate an .cs file.

I have an array of System.Data.DataColumn that i'd like to use as a private variables in my generated code file.

I'm using ColumnName as variable name, Value as variable value and DataType as variable data type.

I'm thinking on how do I initialize defined variables in this case:

ColumnName = "col1"
ColumnValue = "text"
DatType = System.String

I'd like to see output: private System.String col1 = "text";

Variable definition in T4 template:

private <#= DataType.ToString() #> <#= ColumnName #> = "<=# ColumnValue #>"

I'm thinking about writing helper method, that will return variable initialization string for common data types. Something like:

public string ValueInitializerString
        {
            get
            {

                string valueString;
                if (this.DataType == typeof(int))
                {
                    valueString = this.Value.ToString();
                }
                else if (this.DataType == typeof(string))
                {
                    valueString = string.Format("\"{0}\"", this.Value);
                }
                else if (this.DataType == typeof(DateTime))
                {
                    DateTime dateTime = (DateTime)this.Value;

                    valueString = string.Format("new DateTime({0}, {1}, {2}, {3}, {4}, {5}, {6})",
                                                              dateTime.Year, dateTime.Month, dateTime.Day, dateTime.Hour, dateTime.Minute, dateTime.Second, dateTime.Millisecond);
                }
                else
                {
                    throw new Exception(string.Format("Data type {0} not supported. ", this.DataType));
                }
                return valueString;
            }
        }

If someone did something similar, could you advice if this is a good idea or it could be done in more convenient way? Maybe I should read something?

You could also initialize Value as it should be output in the results. Ie: "0" for int, @"""Text""" for text and so on. The compiler will detect any conversion errors when compiling the generated C# code. — Just another metaprogrammer
– Just another metaprogrammer, Commented Apr 24, 2013 at 5:52
The thing is that incoming set of fields, that I use to define variables will change often, so I don't want to see compiling errors. — Sergejs
– Sergejs, Commented Apr 24, 2013 at 8:03

Shawn South · Accepted Answer · 2016-05-20 01:15:40Z

3

+50

This should work fine, although I would make it a generic class. Something like

internal class DataType<T>
{
      public string Name {get;set}
      public T Value {get;set;}
      public Type Type
      {
          get { return typeof(T); }
      }

      public string VariableDefinition()
      {
          /* Construct string */
      }
}

This would more flexible and reusable. Example usage:

<#
var dataType = new DataType<String>()
{
    Name = "col1",
    Value = "text"
};
#>

private <#= dataType.VariableDefinition() #>

answered May 20, 2016 at 1:15

Shawn South

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Comments

SunilA · Accepted Answer · 2016-05-23 18:07:14Z

Hope this works.

Use ExpandoObject to solve your problem. Definition of ExpandoObject as per MSDN :

It represents an object whose members can be dynamically added and removed at run time.

To set datatype and value at runtime , use Convert.ChangeType method. This creates an object of same type and value as you provide.

Reference for ExpandoObject: https://blogs.msdn.microsoft.com/csharpfaq/2009/09/30/dynamic-in-c-4-0-introducing-the-expandoobject/

Reference for Convert.ChangeType : https://msdn.microsoft.com/en-us/library/system.convert.changetype(v=vs.110).aspx

So create properties dynamically using ExpandoObject and create datatype dynamically using Convert.ChangeType.

Code :

class Program
{
        static void Main(string[] args)
        {
        // I have used hardcoded values representing database values
        var dataTable = new DataTable();
        dataTable.Columns.Add(new DataColumn("Column1"));
        dataTable.Columns.Add(new DataColumn("Column2"));
        var row = dataTable.NewRow();
        row[0] = 1;
        row[1] = "Test Value";
        dataTable.Rows.Add(row);

        // This list below contains properties - variables , with same datatype and value
        dynamic parentList = new List<dynamic>();

        var rowOne = dataTable.Rows[0]; 
        for (int i = 0; i < dataTable.Columns.Count; i++)
        {
            dynamic child= new ExpandoObject();

            child.Property = Convert.ChangeType(row[i], row[i].GetType());
            parentList.Add(child); 
        }
    }

}

romain-aga · Accepted Answer · 2016-05-25 21:34:39Z

Define a dictionary:

var _valueConverters = new Dictionary<Type, Func<object, string>>()
{
    { typeof(int), x => x.ToString() },
    { typeof(string), x => "\"" + x + "\"" },
    { typeof(DateTime), x => String.Format("new DateTime({0})", ((DateTime)x).Ticks) }
};

Then write a method like this:

void WriteVariable<T>(string name, string value)
{
    Type typeT = typeof(T);
    if (! _valueConverters.ContainsKey(typeT))
        throw new Exception(string.Format("Data type {0} not supported. ", typeT.Name));
    Write(String.Format("{0} {1} = {2}", typeT.Name, name, _valueConverters[typeT](value)));
}

And call it, like that:

private <#= WriteVariable<string>("col1", "text") #>;

Or even (not really reusable):

void WriteVariable<T>(string name, string value)
{
    Type typeT = typeof(T);
    if (! _valueConverters.ContainsKey(typeT))
        throw new Exception(string.Format("Data type {0} not supported. ", typeT.Name));
    Write(String.Format("private {0} {1} = {2};", typeT.Name, name, _valueConverters[typeT](value)));
}

With:

<#= WriteVariable<string>("col1", "text") #>

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