Setting a variable value using a Script Task in SSIS

Using Script Task, I am trying to set the value of a variable A the value of variable B. Both in the same package.
This is the code I used:

public void Main() {

              Dts.Variables["User::VariableA"].Value =     Dts.Variables["User::VariableB"].Value;


        Dts.TaskResult = (int)ScriptResults.Success;
	}

Both variables are string type, and marked as Read/Write. I tried to convert them to string in the script too using ToString() method, but it didn't work as well.

Anyone knows what did I miss?

You missed nothing. Or perhaps, you've oversimplified the problem for here as a minimal reproduction does not show the same behaviour

I created a package as shown. Two string variables, Variable A and Variable B. They were initialized to "My Value is A" and "My Value is B." I confirm this by printing the current variables to the output window. I then use your code to update the value of A. I then reuse the same inspection code and see the expected final value of "My Value is B" for both variables.

SCR Echo Value

This is the same script task, before and after I update the value. It simply emits the read/write variables to the Information event so they show on the Results/Output tab

using System;
using System.Data;
using Microsoft.SqlServer.Dts.Runtime;
using System.Windows.Forms;

namespace ST_5e898ddfc3e24549a581a83a3cabab4d
{
	[Microsoft.SqlServer.Dts.Tasks.ScriptTask.SSISScriptTaskEntryPointAttribute]
	public partial class ScriptMain : Microsoft.SqlServer.Dts.Tasks.ScriptTask.VSTARTScriptObjectModelBase
	{
		public void Main()
		{
            bool fireAgain = false;
            foreach (var item in Dts.Variables)
            {
                Dts.Events.FireInformation(0, "SCR Echo Back", string.Format("{0}->{1}", item.QualifiedName, item.Value), "", 0, ref fireAgain);
            }

			Dts.TaskResult = (int)ScriptResults.Success;
		}

        enum ScriptResults
        {
            Success = Microsoft.SqlServer.Dts.Runtime.DTSExecResult.Success,
            Failure = Microsoft.SqlServer.Dts.Runtime.DTSExecResult.Failure
        };
	}
}

SCR Assign B to A

This is the supplied code

using System;
using Microsoft.SqlServer.Dts.Runtime;
using System.Windows.Forms;

namespace ST_3a3b3c73d21c472aba3c2ddbad1481b1
{
    [Microsoft.SqlServer.Dts.Tasks.ScriptTask.SSISScriptTaskEntryPointAttribute]
    public partial class ScriptMain : Microsoft.SqlServer.Dts.Tasks.ScriptTask.VSTARTScriptObjectModelBase
    {
	    public void Main()
	    {
            Dts.Variables["User::VariableA"].Value = Dts.Variables["User::VariableB"].Value;
            Dts.TaskResult = (int)ScriptResults.Success;
	    }

        enum ScriptResults
        {
            Success = Microsoft.SqlServer.Dts.Runtime.DTSExecResult.Success,
            Failure = Microsoft.SqlServer.Dts.Runtime.DTSExecResult.Failure
        };
    }
}

What else could it be?

Looking in the wrong place

The Variables window shows the Design-Time value of the package. Perhaps you're thinking that the variable isn't being updated because you're looking at that value.

Instead, if you have a breakpoint or something, look at the Locals window under Variables and that will show what the current Run-Time value is

Express yourself

There's an expression at play. An expression on a variable will always supersede an assigned value. It won't raise an error trying to assign to the Value property but it won't "stick" because the EvalateAsExpression property being true means the Value is always computed based on the Expression property.

In the newer versions of BIDS/SSDT, they make this much more readily apparent with visual cues. The Properties are available via F4 but the Variables window now shows a glyph, f(x), next to the variable name along with pale grey text for the Data Type and Value.