Linq query performance improvements
You can definitely optimize this query. If list has M items and excelViolations has N items, the query will iterate excelViolations M*5N times.

It is possible to reduce this to M*N iterations by consuming excelViolations only once per item in list. You can do this with a subquery on excelViolations that checks all five conditions for each excelVio, then ORs each of the five conditions across all excelVio instances.

If that is not clear, hopefully this walkthrough is:

from vio in list

// Perform all five checks for each excelVio

let checks = excelViolations
    .Select(excelVio =&gt; new
    {
        HasVioID = vio.VioID.Formatted.Equals(excelVio.VioID.ToString()),
        HasRuleType = vio.RuleType.Formatted.Equals(excelVio.RuleType),
        HasVioType = vio.VioType.Formatted.Equals(excelVio.VioType),
        HasXCoordinate = vio.EtchVects.Any(x =&gt; x.XCoordinate.Equals(excelVio.XCoordinate)),
        HasYCoordinate = vio.EtchVects.Any(y =&gt; y.YCoordinate.Equals(excelVio.YCoordinate))
    })

// From left to right, OR each of the five results (equivalent to .Any)

    .Aggregate((left, right) =&gt; new
    {
        HasVioID = (left.HasVioID || right.HasVioID),
        HasRuleType = (left.HasRuleType || right.HasRuleType),
        HasVioType = (left.HasVioType || right.HasVioType),
        HasXCoordinate = (left.HasXCoordinate || right.HasXCoordinate),
        HasYCoordinate = (left.HasYCoordinate || right.HasYCoordinate)
    })

// Filter to only those that pass every check

where checks.HasVioID
    &amp;&amp; checks.HasRuleType
    &amp;&amp; checks.HasVioType
    &amp;&amp; checks.HasXCoordinate
    &amp;&amp; checks.HasYCoordinate

// Same projection

select new EtchVectorShapes
{
    VioID = Convert.ToInt32(vio.EtchVects.Select(x =&gt; x.VioID)),
    ObjectType = vio.EtchVects.Select(x =&gt; x.ObjectType).ToString(),
    XCoordinate = Convert.ToDouble(vio.EtchVects.Select(x =&gt; x.XCoordinate)),
    YCoordinate = Convert.ToDouble(vio.EtchVects.Select(x =&gt; x.YCoordinate)),
    Layer = vio.EtchVects.Select(x =&gt; x.Layer).ToString()
};