Add project files.

src/modules/Elsa.Sql/Services/SqlEvaluator.cs

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+using System.Text;
+using Elsa.Expressions.Models;
+using Elsa.Extensions;
+using Elsa.Sql.Contracts;
+using Elsa.Sql.Models;
+using Elsa.Workflows;
+
+namespace Elsa.Sql.Services;
+
+/// <summary>
+/// A SQL expression evaluator.
+/// </summary>
+/// <remarks>
+/// Initializes a new instance of the <see cref="SqlEvaluator"/> class.WS
+/// </remarks>
+public class SqlEvaluator() : ISqlEvaluator
+{
+    private WorkflowExecutionContext executionContext;
+    private ActivityExecutionContext activityContext;
+    private ExpressionExecutionContext expressionContext;
+
+    /// <inheritdoc />
+    public async Task<EvaluatedQuery> EvaluateAsync(
+        string expression,
+        ExpressionExecutionContext context,
+        ExpressionEvaluatorOptions options,
+        CancellationToken cancellationToken = default)
+    {
+        if (!expression.Contains("{{")) return new EvaluatedQuery(expression);
+
+        expressionContext = context;
+        executionContext = context.GetWorkflowExecutionContext();
+        activityContext = context.GetActivityExecutionContext();
+
+        // Create client
+        var factory = context.GetRequiredService<ISqlClientFactory>();
+        var client = factory.CreateClient(activityContext.ActivityState["Client"].ToString(), activityContext.ActivityState["ConnectionString"].ToString());
+
+        var sb = new StringBuilder();
+        int start = 0;
+        var parameters = new Dictionary<string, object?>();
+        int paramIndex = 0;
+
+        while (start < expression.Length)
+        {
+            int openIndex = expression.IndexOf("{{", start);
+            if (openIndex == -1)
+            {
+                sb.Append(expression.AsSpan(start));
+                break;
+            }
+
+            // Append everything before {{
+            sb.Append(expression.AsSpan(start, openIndex - start));
+
+            // Find the closing }}
+            int closeIndex = expression.IndexOf("}}", openIndex + 2);
+            if (closeIndex == -1) throw new FormatException("Unmatched '{{' found in SQL expression.");
+
+            // Extract key
+            string key = expression.Substring(openIndex + 2, closeIndex - openIndex - 2).Trim();
+            if (string.IsNullOrEmpty(key)) throw new FormatException("Empty placeholder '{{}}' is not allowed.");
+
+            // Resolve value and replace with parameterized name
+            var counterValue = client.IncrementParameter ? paramIndex++.ToString() : string.Empty;
+            string paramName = $"{client.ParameterMarker}{client.ParameterText}{counterValue}";
+            parameters[paramName] = ResolveValue(key);
+
+            sb.Append(paramName);
+            start = closeIndex + 2;
+        }
+
+        return new EvaluatedQuery(sb.ToString(), parameters);
+    }
+
+    private object? ResolveValue(string key)
+    {
+        return key switch
+        {
+            "Workflow.Definition.Id" => executionContext.Workflow.Identity.DefinitionId,
+            "Workflow.Definition.Version.Id" => executionContext.Workflow.Identity.Id,
+            "Workflow.Definition.Version" => executionContext.Workflow.Identity.Version,
+            "Workflow.Instance.Id" => activityContext.WorkflowExecutionContext.Id,
+            "Correlation.Id" => activityContext.WorkflowExecutionContext.CorrelationId,
+            "LastResult" => expressionContext.GetLastResult(),
+            var i when i.StartsWith("Input.") => executionContext.Input.TryGetValue(i.Substring(6), out var v) ? v : null,
+            var o when o.StartsWith("Output.") => executionContext.Output.TryGetValue(o.Substring(7), out var v) ? v : null,
+            var v when v.StartsWith("Variable.") => expressionContext.GetVariableInScope(v.Substring(9)) ?? null,
+            // OBSOLETE: This is deprecated and will be removed in a future version. Use 'Variable.' instead.
+            var v when v.StartsWith("Variables.") => expressionContext.GetVariableInScope(v.Substring(10)) ?? null,
+            _ => throw new NullReferenceException($"No matching property found for {{{{{key}}}}}.")
+        };
+    }
+}