Original Snippet:

    private void radButton1_Click(object sender, EventArgs e)
    {
        string perp = radTextBox1.Text;

        int i = 0;
        DataRow arp = ale.Rows[i];
        while (i <= ale.Rows.Count)
        {
            if (ale.Rows[i].Field<>("FullName") = perp)
            {
                arp = ale.Rows[i];
                ale.Rows.Remove(arp);
            }

        }

        i = ale.Rows.Count;
        radLabel1.Text = i.ToString();
    }

Changed this:

    if (ale.Rows[i].Field<>("FullName") = perp)

To This:

    if (ale.Rows[i].Field<String>("FullName") == perp)

Got the same error (fresh installation of the VS2015 Enterprise, ASP.NET webforms project .NET 4.0).

After some investigation I've found that there are two DLLs in references which causes this. Both are .Net 2.0 assemblies and both of them obfuscated by Red Gate Smart Assembly 6.5. And the real reason is... obfuscation.

Luckily, these assemblies are mine, so I've tried to build them without using of Smart Assembly - error is gone.

Interesting is that no any errors or warnings shown by Visual Studio before trying to build a project.

Good luck!

EDIT: Updating Smart Assembly to version 6.9 fixed an issue.

As @Andrey reported this does appear to be an issue with obfuscated assemblies that is causing some difficulty for Roslyn. Today I was able to get a live repro of this error and the root cause appears to be the obfuscator is invalidating / corrupting how default parameter values are stored in metadata. When run through ildasm the default values will be displayed as:

.param [3] /* Invalid default value for 0800001F: */

The previous version of the compiler handled this scenario by treating the invalid value as null or default(T). We will be fixing Roslyn to have the same behavior.